the two white cars are there because it's not possible remove it at all from gp4.
they uses the original gp4 prost carshape with a set of all white textures
you can replace it with every fantasy car you want, but the shapes needs to be called car_prost_car1(2)_lod0(1234).gp4. textures name can be prost_1.tex etc.. or any other name already not used by any other car.
just pay attention that the arrows name (both for carshape and textures)
is already used for the toyota.
from gp4italia site are available a patched 9.6 exe and an update for the cars.wad (this one with a new set of tyres and some shape bugfixes)
bye