I'm going to expand on what Almost610 said.
There is a very strict set of guidelines here regarding backwards conversions. If we put IV on a old engine or in a cheap game, Rockstar loses money, because why pay $30 when you can buy a used GTA for a few bucks and then mod the IV stuff in? Beyond that, the III engine is not really capable of the kind of physics and capabilities IV requires, not to mention its rendering limitations, so it probably would not work anyways. Either way, it's against the rules and the sad fact is that few people care about III enough to try to put IV in it.
You can post in that request topic if you want, but you won't get any help. Sorry dude.