Before you do anything, copy the GP4.exe file. Paste it somewhere outside your GP4 folder and rename it GP4_old.exe.
Make sure you have updated GP4 with the 9.6 official patch. Also make sure you have the GP4 NoCD patch that will work with the 9.6 official patch. That will patch the GP4.exe file so that you can play the game without the CD and update the game. Allow the NoCD GP4.exe to overwrite the original GP4.exe that comes in the 9.6 patch
Download gp4master.
Now that you have updated you game to 9.6 and are using the GP4.exe patch with NOCD, move your GP4_old.exe file back into your GP4 folder. After installing GP4 master use GP4_old.exe to get GP4 master to work. When it asks you to locate GP4.exe, locate GP4_old.exe instead. This is the only way to get it to work as far as I know.
GP4 master can be a bit confusing. what you have to do is as follows. After installing GP4 master and getting it to run properly go to the third box from the right. It has a little symbol with a small number 1 on it. Now go to load WAD-file and locate the "cars.wad" file in your GP4 folder. Load this file. Once it is loaded click on "extract all files" which is down the bottom. Select a directory to extract them to (C:/Program Files/GP4/Car Wad File for example) and click ok. Now all you have to do is download all the cars and carshape files you want to put into the game. Once you have downloaded them open them one by one and take all the files out of the folders and place them in the cars.wad folder that you just created. When you open the folder there will be three other folders. 1.Cars, 2.fonts, 3.maps. Paste all you downloaded car files into the car folder. Obvioulsy you don't have to paste the readme.txt files and jpeg files from the cars that you downloaded. Allow all your downloaded files that you are pasting in your car folder to overwrite the originals. Now all you have to do is go back to GP4 master. Go the same place you were in to extract your cars.wad file and click "Create wad from directory". Now choose the directory that you extracted your cars.wad file to. (C:/Program Files/GP4/Car Wad File for example). Click OK. It then asks you to name the file. name it cars.wad. GP4 master will then create a cars.wad file.
PLEASE MAKE SURE TO BACK UP YOUR ORGINAL CARS.WAD FILE. Name it cars.wad.bak for example so you know what it is and it doesn't interfere with the other .wad files. If your new cars.wad file doesn't work properly you can always revert back to this one.
Now move your new cars.wad file into the location that the original cars.wad file was. Your done. Play GP4 and enjoy. I hope it works! Sorry if this was confusing but the whole thing is a bit confusing! Took me a while to work it out for myself. There are some other handy things you should download as well like GPxpatch. make sure it is the latest version. Also Team Editor 1.0 is very good.
You will find that some downloaded cars actually install themselves, but so far I think only Viper does this. His cars are pretty good.
Q*GO NICK HEIDFELD!!!*Q