Whilst I agree now that GTA IV was slightly overrated, the game is also MASSIVELY overhated on this forum. Your fault with disliking GTA IV is due to your love of San Andreas - nothing wrong with feeling that, but it has clouded many people's judgement in rating GTA IV.
And why will GTA V not suck? Just look at every piece of information, screenshot and trailer released.
I just keep thinking 'Skyrim.. Skyrim.. Skyrim..'. IMO: horrible game. And that was at least 7 steps forward since Oblivion; but Oblivion was - and still is (in my opinion) the better game.
Same could be said about GTA 5, but then again.. It could just blow our expectations out of the water.