Any game can be taken advantage of, just as real life can be. But I think GT4 had a great formula, and I think it reflects on the economy of real life racing fairly well. Starting off, you're stuck with buying used cars and perhaps not modding them up at all. As you progress, very gradually, things open up for you, till at the end, you have almost more money than you know what to do with.
Polyphony can do any number of things to fiddle with the Gran Turismo formula. They can have prize cars be of lesser potential and need to be built up for later races, they can eliminate prize cars entirely and give bigger prize purses, leaving us to buy whatever cars we want, they can add in maintenance and fees, they can make damage expensive to repair, all sorts of things.
Personally I'm hoping big time for some version of our Career Mode, so that those who want a more simulation of reality game can dig seep into that aspect, and deal with the world of racing based entirely on realistic principles. Meanwhile, those who want to pursue GT as it was originally built can take on Gran Turismo Mode and play the traditional game with prize cars and cash for races won. Career Mode would put you in a tightly structured and scheduled season, so you wouldn't be free to chase some money races over and over to amass quick wealth.
In Forza, there's a pretty well connected sequence which runs for quite some time in the game, where a race win will give you a decent cash prize as well as a car which can be used in another race. Following this chain, you won't have to buy a car for quite some time, saving money for mods and performance improvements. This can be milked to a great extent, as money piles up quickly. In my second profile, I've gone through less than a third of the game and I already have a million credits. Plus there are true cheats and glitches on top of successful racing.
With GT4, it seems that you can't really cheat the game into amassing wealth until later on in the game, and even then, it's simply a matter of doing high prize yielding races, which pro racers would be doing anyway, so I wouldn't really call it cheating. Maybe once you win a race, it can't be raced again until you complete a series of associated races, but I don't really see a need for that, personally. I think GT4 as it stands is pretty darn good.