This is my Forza vs GT4 comparison based on what I've seen, read or listened on the Internet, by each comparison point I find important:
Graphics:GT4 looks a notch better in screens but this may be related to the fact they show almost only Photomode pics and also because Forza supports damage, wich influences the amount of polys you can put.
Winner:GT4
Gameplay:GT4 is terribly good, very close to life but Forza is not bad. Comments I saw range from "superior to GT3" to "equal to GT4". No FFB on the horizon for wheel players tough.
Winner: GT4, by a small margin.
Car count: Discussion ends here:GT4 will have 650+, but Forza will have 200+ and could come close to 500, according to some sources, not bad!
Winner:GT4
Car lineup quality: GT4 sports many nice rides and race cars, but they lack some important ones and their love for 50 hp lame cars and 20 Skyline variants harms them.
Winner:Forza
Track count: Gt4 will have many, including some real life legends, but Forza promises to get some good ones too.
Winner:GT4, lack of info from Forza
Car tuning: Forza will have something for anyone:real upgrades for real drivers and ricer kitsfor NFSU wannabes, GT4 will have good tuning, but no body kits or paint mods announced.
Winner:Forza
Online: Let me do it short:Forza will have online with 8 cars and we know it will work well, Xbox live has made its proofs.GT4 will have it, but only in a year.
Winner:Forza
Sorry for the long post
See ya