Sure. Why not. As the Gran Turismo team are already dedicated to making the games as realistic as possible, I'm sure they would very accurately re-create its effects (unlike arcade games such as NFS Underground, where it seemingly gives you light-speed and makes the world go blurry). And, since nitrous oxide systems are, like it or not, a very realistic tuning possibility in the real world, having an accurately re-created system in GT4 would make the series even more true to life.
If it were to be put in, it would need to be a very complex vehicle modification, requiring you to have real knowledge of how to correctly tune such a system in order to gain any benefit from it. Like the Professional suspension system - it does you practically no good unless you know how to properly tune it. If they made it a simple "bolt-on-and-automatically-get-100-extra-horsepower," then it would suck.
Nitrous oxide has very real benefits in the world of racing, especially drag racing. From the screenshots I've seen of GT4, it looks like drag racing has been incorporated. If this is the case, a properly tunable nitrous injection system would be a welcome addition.
Also, it would be excellent if they could incorporate the option of nitrous lag reduction systems into the game. Simply using small amounts of nitrous near the bottom end of the rev range to counter turbo lag - again another very realistic scenario. While racing around a track at high revs, it would be virtually not used at all, but would give the car a little extra help when lugging out of that tight 120 degree 2-gear turn...
So I give a PROPER Nitrous Oxide system the thumbs up for the GT series, and give "press-a-button-for-a-berzerk-mega-speed-shot-to-steal-the-race" nitrous systems the thumbs down for the GT series.
BTW, "real" racers often do use nitrous...it's no more of a "cheat" than a turbo, or even disc brakes. The reason many racing series don't use it is because of heavy regulations used to keep all team's engines running at about the same level. The comment that "real racers" don't need it is as ludicrous as saying they don't need disc brakes...