According to IGN, Gran Turismo 4 is complete. Perhaps rumors of additional delays are premature?
GT4 includes over 700 cars ranging from historic models through model year 2005 and even concept cars and over 50 courses including Fuji Speedway's 2005 configuration.
The offline-only version may not be released in all territories, suggesting that the online version may be released sooner than many anticipated.
In Japan, GT4 will be available in a limited edition Nike bundle including a limited edition Nike T-shirt and a pair of Nike Shox Total Magia II GT Limited Edition shoes. Additionally, this version has EyeToy support...if you hold this Nike T-shirt in front of the EyeToy, you will unlock a special Nike One car...designed by Nike.
Polyphony Digital is working on "Gran Turismo for Boys" for PS2, a simplified (and budget priced) version of the driving game for young kids.
Polyphony Digital is still hard at work on "Gran Turismo 4 Mobile" for PSP.
Gran Turismo 5 will be on Sony's next-generation console and will feature overturning cars (with each vehicle's underside accurately replicated) and, more significantly, collisions featuring shape deformations based on physical calculations. Also mentioned is some form of image recognition feature (perhaps not unlike what they're doing with the EyeToy in the Nike bundle), but no details have been revealed.