My opinion on the sound in GT4:
Most of the cars sound pretty awesome. But I must say, some of the Le Mans cars sound pretty disappointing. The best of them are the Pescarolos, the Mazda 787B and the Viper GTS-R. The GT-One, Sauber C9, Minolta-Toyota, Audi R8, Bentley Speed 8, BMW V12 LMR and McLaren F1-GTR all use old, unrealistic engine noises from GT2 (dunno if they were in GT1 as well)! I don't like how all three Nissan race cars have that same, unconvincing engine noise. The Camaro LM sounds like a Mazda Miata. The Ford GT LM Race Car (both Spec I and II), Corvette C5R and Mercedes-Benz CLK-GTR sound like most of the German touring cars, which don't sound very good. The other imaginary LM cars have decent engine noises.
Most of the JGTC cars sound brilliant, but a couple of the Supras and NSXes have that disappointing touring car noise. The Calsonic Skylines (from 1993 and 2000) sound exactly like the Audi R8 and Bentley Speed 8, with that old PS1 era engine noise. Most of the other race cars and rally cars in the game sound good as well (I like how the rally cars sound so barking mad).
The road cars sound almost as you would expect them to. Some of them are weak, like the Plymouth Superbird. Some of the supercars don't sound very good, especially the super-concepts. For instance, the Tommy Kaira ZZII has an old engine sample, and the Cadillac Cien, Audi Le Mans Quattro and the Volkswagen W12 are somewhat annoying. The Ford GT sounds weak, and the SLR, well, I have no idea where they got that engine noise from. But both cars' engine noises sound considerably better with a racing and sports muffler respectively. The worst sounding cars in the entire game are the Chrysler Crossfire, which sounds like an accordion, and the BMW 130d (Its a diesel. Need I say more?)
If PD couldn't get all the engine noises for each car because of time constraints, then I can forgive them for that. But they took each car and meticulously recorded their performance characteristics and looks. So I can't see why they couldn't record the engine noise for each car.
I've heard the engine noises in the Vision Gran Turismo tech demo, and the GT4 intro. They are absolutely fantastic. So I'll be expecting the cars in GT5 to sound just as impressive.