GTAIV's framerate is absolutely terrible when anything is going on. GTAV is generally unaffected by things like gun fights, cop cars, etc., with the exception of explosions & fire.
There are a few key regions that run lousy: The northern highway up near.. Grapeseed or wherever, when you're going through the mountains in particular.
Doing the "optimal install" -- installing mandatory disc to HDD, installing play disc to a USB, launching from the pins, really does make a noticeable change.
Also, the two games are so vastly different graphically they're barely comparable.
...and GTA 5 also has a blurry FXAA filter over the graphics, which gives the game a very soft look. Red Dead and GTA 4 look a lot sharper and clearer in comparison.
Are you sure we're talking about the same game, on the same console?
GTAIV on 360 is the blurriest mess of a game, it aged horribly. You can't read street signs even right below them.