did you ever try it step by step ???
1. extract cars. wad to a directory.
2. change nothing (!)
3. make a cars.wad file out of your directory.
4. try if it works.
5. if it doesnt, come to forum for help, something is completely wrong then.
6. if it does work, change somefin in the cars.wad-directory. maybe a new steering wheel, a new livery for one car... but only ONE thing at a time.
7. make a new cars.wad. try it. if it doensnt work, come back to forum.
8. if it works, try other downloads, until you reach the update that causes your gp4 to crash. then come back to forum.
dont give up after just one week or so. gp4 needs patience.
greetings, jp