To be honest, GTA 4 ha a much detailed city. You can see props lying even on the streets - a feature removied in GTA 5.
The good thing about GTA 5's graphics are the new graphical effects like the much cooler lighting. It fails on Anti-Aliasing and on the LOD part.
To be honest, Far Cry 3 makes GTA 5 look like a game from 2006.
Ubisoft did a good job on this, but so did R*. Do you know why?
Ubisoft took advantage of the "less objects spawning" characteristic of FC3. It used the HDD more than it uses the RAM. And for that, they've allocated more of the GPU's work to the RAM because it doesn't have to remember a lot of objects 'cause the HDD is already handling that. The only thing that the RAM is handling in Far Cry 3 are the random events and the event nodes.
In GTA V's case, this is partially false. GTA V makes the HDD/SSD remember all the "static objects" such as the map and the props - just like Far Cry 3. But, it's VERY RAM-heavy because it has to spawn at least 5 peds EVERY TIME in almost EVERYWHERE in the city. Heck, even in the countryside. Not just peds, but so does the wildlife, the traffic, the AI, and the random events.
FC3 spawns much less peds, traffic, and wildlife at the same time, hence it doesn't overload the RAM.