The whole universe of Grand Theft Auto is conventionally divided into three unequal parts. The oldest games belong to 2D Universe. Starting from GTA 3 until GTA San Andreas - 3D Universe. And finally, the last two games of the series - GTA 4 and GTA 5 are in HD Universe. These last two games are not in vain in the same category. They are really very similar. Games have the same physics engine, the main feature of which is that the calculation is carried out anew in each case. However, physics in the two games have significant differences. In particular, it concerns body movement. Meticulous players made a special video with comparison of GTA 4 and GTA 5:
The video clearly shows that in GTA 4 Rockstar Games just tried the new engine, assessing its possibilities and potential, and then in GTA 5 has used all the experience. Of course, not everything was left unchanged. Some things are likely to have been specifically simplified for better optimization or for some other reason (like in order to start the game on the PlayStation 3 with its 256 MB RAM). It's hard to say if it's bad or good, because in GTA 4 for that slightly more realistic physics you will have to deal with 7 times smaller game world, lack of online mode, worse graphics and other cons. Besides, there are many mods for GTA 5 that will make the physics of the game at least not worse than in GTA 4.
Speaking about the melee combat system and the behavior of people in the game worldw we should consider the case more clearly. Another video undertaking a detailed comparison of the key gameplay elements in GTA 4 and GTA 5. The differences become even greater. A particular advantage of GTA 4 are the car physics and more realistic explosions (although this is subjective opinion of our Analytical Department). Also, in GTA 5 you can pick up and throw objects, although the need for such functions in the GTA game is highly questionable. We should also mention the interactivity of many buildings in GTA 4, which we don't see in GTA 5. Although, the 7 times more massive game world must somehow compensate it.
To summarize, we can say that GTA 5 is definitely better and bigger than GTA 4, despite the simplification of a small number of game elements. Don't forget, also, about the GTA Online and it's regular updates with tonns of content. Meanwhile, GTA 4 can't offer the player anything more than it's main story and two not-so-big (though very high quality) DLC for it. Not exactly a correct approach when creating an open world game, right?