GTA IV makes you feel like you're apart of the game. GTA V feels like a movie being directed. You can't even do missions without that horrible checklist coming up making you feel as if you should be playing R*'s way.
Simple as that IMO.
They both feel like movies, but V feels like a Michael Bay 2 hour movie that you're forced to sit through, yet IV comes across as a Scorcese/Coppola six-hour epic that the producers have given you the directorial reins to. The difference between a Mickey D's cheeseburger (V) and a prime cut of Filet Mignon (IV).