For a fair comparison, your gameplay comparison is rather lackluster don't you think? You skim over what was improved in order to make GTA V look better.
GTA IV was not just an improvement upon the 3D universe gameplay. GTA IV was a restart from scratch. It was a new foundation. It changed everything.
The basic movement system was vastly improved with the addition of physics, grabbing ledges and moving along them, picking objects up without a prompt, and the physics integration.
The physics alone added a ton to the game that wasn't there before. Both to the movement and to the combat. Vehicular combat in particular now has real-time reactions rather than scripted death animations. Pedestrians and even the player will ragdoll pretty realistically, but perhaps not perfectly realistically, roll over or under the vehicle as it hits them. They will react from the area they were hit when hit with a thrown object. It was a vast change from the past games.
Combat was changed in a variety of ways. The hand-to-hand while still clunky and imperfect was an improvement from San Andreas with additions like dodging and disarming. You could dynamically turn trash into a weapon by picking it up and using the physics engine. The gunplay was inspired by games like Gears and Resident Evil 5 with the over-the-shoulder camea. The cover system is dynamic and allows you to take cover pretty much anywhere, even with moving objects. This allows you to do much more than in the previous games with the combat.
The phone, whether you liked being called or not, was a vast improvement to the previous cell phone which did not exist except to receive calls. It opened up a lot of freedom such as calling 911 or finding tidbits of story by randomly calling up your buddies after a mission to see if they had anything to say. It improved the mission variety with a semi-calendarish app to set appointments on (which were admittedly only used in two or so missions but it's still used.
A friend system was added that was a very nice addition with some potentially major flaws. The calls in GTA IV are inconsistent, sometimes coming at frequent intervals and other times coming in long intervals. I myself usually am only called 2 or so times per hour while other people have reported annoyingly frequent calls. The friends are also very picky. While it would make sense to have one or two friends be very particular about you choosing not to hang out with them (Dwayne for instance due to his depression), most should be rather understanding when something comes up. Overall though, it was a fun addition that needed work.
The radar was brought up to speed with the improvements made in other games like Saint's Row and gave us a line to our destination rather than just a dot. The HUD itself was shrunken to its base elements to save space and to give the game a more realistic feel.
The police system was changed heavily from the previous games. Whether you like them or not the cops are now no longer completely mindless and following you wherever. They actually search for you, although not perfectly. We no longer needed to go to the pay n sprays to lose our wanted level. This was a huge change to the dynamic of the game and you don't even mention it in this 'fair' comparison except in a separate section. What? The police don't affect the gameplay now? You also speak of the LCPD being inescapable at 3 stars, but that's just idiotic as it's something that has been done multiple times and I'm pretty sure is required of the game. If you mean escaping in a helicopter then you're pretty much SOL because it's not hard to follow a dang helicopter with another helicopter. Use some common sense.
Taxis are also made useful in GTA IV which is something that was sorely needed in GTA. It may be fun to travel in the games but even in GTA V I started using the taxis a lot after it got tiring.
Driving: Whether you want to include it or not, was also vastly changed in GTA IV. The game strove for a sense of realism but didn't quite hit the mark so it ended up feeling floaty. They did, however, give us the ability to control our speed, made it so we had to slow down to make turns, and felt completely different than what we were used to. Personally, I could feel the different vehicle weights which was amazing and made the driving feel great. I won't say it's better than V but I will say that it was a step forward from San Andreas.
Side content: Being the first of its kind, GTA IV was short on several side missions that GTA had in the past. It didn't have no side missions however. Things like drug deliveries, car theft, vigilante, QUB3D, dating, friendshipping, playing darts, playing pool, and playing bowling were all added and were nicely done.
The combat: Updated from GTA IV. This is one of the things that I really love about V. It has some flaws like all three characters essentially fighting the same way, but the fighting itself is not bad. We're not looking for Sleeping Dogs level of fighting combat in GTA. The shooting has been touched up and feels closer to Max Payne than it does GTA IV. It's a nice touch. The cover combat feels essentially untouched. The weapon variety has been improved and the ability to upgrade our weapons with flashlights and more ammo is very nice. However, we do lose the ability to pick up and throw objects, which was a dynamic element that really made GTA IV's combat fun and at times humorous. Another terrible addition, which may be fixed with patches if it hasn't already, is that enemies can headhshot you. At full health without armor I've been killed in a single hit and that is just ridiculous. It hasn't happened in a while though, so maybe it was patched.
The health system has been changed into a bastardization of Red Dead and GTA IV's health system. It takes ideas from regenerating health-based titles without taking the regenerating health all the way. We end up to the point where most of the time we might as well not even have the regenerating health because we only take a few hits to die. It's a strange and awkward addition that sometimes left me feeling cheated.
The basic movement has lost a bit to be honest. Pedestrians don't interact quite as much with you in terms of physics unless you full on sprint them down. Walking feels less dynamic. The actual movement has been improved from GTA IV, however, so it's not like this is a completely botched thing. Climbing has been stripped down so that we can only climb where Rockstar want us to climb. Ladders have been added so we can get to the roof, but it isn't much of a replacement for being able to find ways up to rooftops that Rockstar never intended us to use. Sprinting has taken a huge turn for the better as Rockstar gives us the best take on sprinting in GTA ever. The rest is pretty much just like GTA IV that I can tell.
The phone is upgraded to be more modern and we have the ability to take pictures and browse the web. There are no more multi-day missions so the schedule or calendar or whatever is gone. Instead the phone can now take pictures and browse the internet to replace internet cafes. It's a nice phone system overall, though they could have added more dialog for it and the dial pad could be more obvious.
The friends system has also been dumbed down to its bare minimum. Rockstar didn't fix anything about it in favor of removing everything but the activities themselves. Friends offer no reward anymore and don't like/dislike you. You can just choose to hang out when you want to and they never call you. While we don't want calls every 5 minutes, it was a nice touch to get a call every now and then so actually improving this would have been the preferred route. Especially since we no longer can earn any neat features from it.
The radar has been made square and 3D. It's a nice change but it's still a mini-map with the same basic features as GTA IV's. The visuals are a nice change though and the square version shows more of the city, so it's definitely a functional change. Feature-wise, though, it's basically the same. Speaking of the map, the actual map now allows you to mark places of interest, which is a nice feature.
The police system has been reworked in ways that both are nice and infuriate me. For starters, it takes too long for them to stop hunting you, leading to moments where they catch you at what feels like the last second and you have to start the whole chase over again. The police themselves are almost too good of a shot. Combine this with that they still have some elements of x-ray vision from GTA IV and can spot you in an area they have no business seeing you in (like while standing on top of the covering of the gas station and they're under the gas station and see you without a helicopter). I have only ever escaped above 4 stars once due to the difficulty. I'm not saying they're going to be difficult for everyone, but I found them more difficult. It's also better to take the word of the majority than your own experience in this case. "I didn't experience it so it's obviously the way I see it!" is not a fair comparison. If a lot of people say GTA V's cops are harder than IV's and are too hard then maybe you just sucked at IV and are better at V while everyone else is the opposite?
Taxi's in GTA V are worse than the ones in GTA IV. In IV they were frequent likely due to NYC having more cabs than LA, however if you needed to call for one it would instantly appear on the mini-map and you could work your way towards it. In GTA V they don't show up on the mini-map until they stop and beep for you to get in, so you end up having to wait around for a surprisingly long period of time for the cab to show up and sometimes they don't show up at all (which I think could happen in GTA IV too). While I do like that the default button press is now used to ride the taxis, I don't see why they stepped back in seeing it when requesting it.
Driving in GTA V has been overhauled to be more like San Andreas, but still retains plenty of feeling from IV. It feels almost like we traded one side of the coin for the other, as the cars don't feel consistent in weight. While I could always tell the difference between a sports car and a normal car in IV, in V sometimes a normal looking car feels like a sports car or a large truck feels like a small pickup. I should not be able to chase a fast train in a big truck. Why do sports cars get better off-road performance than off-road vehicles? There are a lot of things that could be improved in V's driving. However, it is still fun driving, so it's not necessarily bad.
Side content: Oddly, GTA V deletes pretty much all of IV's side content, except darts, and replaces it with its own. We now have tennis and golf, we can still take friends out but it's been made more pointless, we can buy properties (which feels a little rushed), there are parachuting, rampages, flight school, and bounties. What is kinda striking is that while GTA IV was setting up the basic gameplay and has reason to be scarce or at least simpler with its side content, a lot of the side content in GTA V is shallow as the content in San Andreas. The properties feel rushed and there are only 4 bounties and 4 or 5 rampages. Given all the time that GTA V had to improve upon GTA IV and to add new things, it just seems lackluster. Not that the side activites are not fun, they're just not of the same quality that I was expecting from GTA IV. What little IV did was done very well and what big things GTA V did were either done before (Base jumping from TBOGT so Rockstar had that made in 2009) or were done awfully (properties are both annoying and IMO more intrusive than the friends mechanic in GTA IV). Overall, it's still strong in this area but in a different way.
GTA V definitely has a lot more variety to the vehicles, but IV was the first of its kind so it's kind of like saying "Why didn't GTA III have the variety of San Andreas?" It's not a huge victory and the fact they got rid of any vehicles from GTA IV hinders that victory. Even if it has better variety there was no reason to get rid of stuff from GTA IV.
You really, really, really like wording your post to paint IV in the harshest of light. Where is the fairness in your comparison? GTA IV set up the basics for the weapons and it left it up to the sequels to bring the bigger and better weapons. Just like GTA III did. After four years from the expansions of GTA IV, the least we can expect from V is more weapon variety. It's not a massive victory. V is, after all, the successor.
Personally, I can't have fun free roaming in GTA V even with these weapons because the police system is too damn annoying and the health system is damn annoying and the lack of basic features from iV is annoying. What IV got right wasn't the additional gameplay features but the basic gameplay. That's also kinda where GTA V messed up for me.
Story: You can't ignore the story just because you want to. This is a fair comparison after all.
GTA IV and GTA V are two entirely different kinds of stories.
GTA IV is a slower story focusing more on the harshness of life and crime. At times it feels too slow in that Niko doesn't really do a lot of searching for his goal, but it features a large variety of interesting characters and moments that sometimes reflects real life too closely. It's a wonderful and yet somewhat flawed story that sometimes goes against the game its part of and other times embodies it. I haven't found many better stories in gaming than GTA IV. TLAD was a weak attempt that I felt was rushed and should have been longer so I would actually get attached to the characters and TBOGT just didn't even seem to try and felt like Rockstar just gave up and wrote whatever.
For me, GTA IV is THE game to turn to if I want to enjoy a good story.
GTA V is a fast paced action comedy. If GTA IV is the "Godfather of gaming" then GTA V is the Rush Hour done with the villains as the main characters. It has a variety of darker moments, but the overall tone of GTA V is fun, fun, fun. Not over-the-top wacky fun like Saint's Row 4 but just a pure kind of fun that you can't get by going overboard. What's better is that it retains a lot of the depth that GTA IV introduced without sacrificing this fun. It's a breath of fresh air and while it's not perfect, it's something different and something fun.
In the end both games have different strengths and weaknesses. GTA V is disappointing not because it is a bad game but because it could have been... no should have been so much more. GTA IV was the GTA III of the HD era, which gives it a fair excuse for a lot of its problems. The expansions then improved the gameplay experience slightly, giving Rockstar even better of a base to build off of, and GTA V should have taken good advantage of that. Instead it feels like a missed opportunity. Rockstar seemed to mess with things that didn't need messing with, stripped down features instead of fixing them, and threw in as much stuff as they could to please the people who wanted more stuff in GTA IV.
What GTA V really feels like is a game that they started out of love, with big ideas, and finished out of requirement. Next-gen probbaly forced their hand to rush GTA V to the market, which shouldn't have happened after four years of work. Maybe it was GTA:O but I doubt it. Maybe Rockstar ran into problems that they couldn't fix, but we don't know. Maybe they didn't start working on it until late due to the desire to plan everything out, but we can't fix that.
Neither game is bad. Neither game is perfect. What you desire out of your game will drive what you feel. I find the core mechanics of GTA IV to be plain fun and can just jump in and play it without needing the extras of V. For me, V is more boring because I don't like the basic mechanics as much. If you need miniguns and stickybombs then GTA V will likely be your preference.