You can install loads of tracks... most NR2003 tracks will work with a simple edit. You need to add a line to the track.ini file (or download an already altered one for that track), and copy 5 setup files across from any of the existing GTP track folders.
There's a lot more than 4 AI cars. The problem is there's only 4 AI chassis type (effectively Sauber, Jaguar, Toyota and Mazda, before iRacing.net got involved). There's skins for more than 4 cars though, and you can download additional cars from sites such as
[www.gtpzone.com]GTP comes into its own in endurance racing though... you'll know this if you've been following our monthly VTR endurance race threads... there'll be a rather big one popping up this coming weekend... Le Mans 24hrs.