Here is a good news, opposite to GP3, GP4 Car (and track) Textures can have bigger sizes than the original ones.
I was already knowing it about Tracks (some of Bruno's track textures are bigger than originals) but i just tryed with Cars textures, after having work on texture for CamEditor (Yes, i have now textures in Cameditor
Here is the result:
Think it would be interesting to see what Ralph Hummerich's team could do with 1024*1024 textures
A last word to say that biggest size texture also slow down the engine and use bigger size of many textures could be another source of slowdown, so will need to use them with care...