The PAL version of the game has the following cars that don't exist in the NTSC U/C version:
Toyota Esso Ultraflo Supra (JGTC) '01 (due to the dispute between Exxon & Standard Oil about the Esso/S.O. trademark when heard in radio adverts in USA.)
Vauxhall versions of the Opel cars. (VX220 / Speedster & the Astra & Calibra Touring Cars)
There are also different naming conventions to reflect local markets for some models, e.g.:
Nissan Silvia K's Aero (S14) '96 in PAL is the same as the Nissan 240SX (S14) '96 in NTSC U/C,
Honda V6 Ima Mid Dualnote '01 in PAL is the same as Acura DN-X Concept '02 in NTSC U/C, etc.
I'm not going to list them all,
Famine put together a spreadsheet that lists all the versions of cars that're available in each different release of GT4 (There's several different regional NTSC versions!)
There are also some proven power / performance differences between similar cars in the 2 versions,
GT4 WRS Week 51 : "Re-start your engines"
pal versus ntsc versions of gt4: what are the differences? and some more info in here.
Also there are differences in the some of the licence tests and driving missions, most notably that it's widely acknowledged that the pesky Mission 34 is easier in PAL than NTSC U/C.