1. Get carset creator, you can download this from the main site.
2. Select the cars tab and import all
3. The teams are numbered 1 - 11, pick the team you want to edit and "Save TGA".
4. Open PSP and edit away.
to import:
repeat steps 1+2
pick your team and "load TGA"
export all
load GP4 (using gpx for best game experience) and enjoy
btw
Welcome to the frustrating world of GP4 editing