Check if the lods from 1 to 4 are copies of the lod 0. In that case if you use CSM you can delete them and set the CSM to use only the "lod 0". This can lower the weight of the wad a lot.
Another thing you can do is see what textures each car carries. So that there are no textures in the wad that are not used.
Many times when you open a wad you find files that are not used (including I have found Photoshop files ...).
You can check the wheel lods, and tire textures that you do not use.
If the mod is for CSM, it is easier to check if there is something that is not used.
With Slimtex from ZAZ Tools you can lower the size of the textures by dragging them.
If you want, pass me the wad and I can see it.
Mods & Tracks for GP4 - [www.mediafire.com]Keeping alive the GP4