Replace the .wad in the GP4 main folder with your track's custom .wad file.
Replace the .dat in the GP4\Circuits folder with your track's custom .dat file.
You can do that with all tracks, just be very careful that you can only replace tracks with the same name - custom tracks usually come with files that are named accordingly to one of the original files. For example, F1virtual's Bahrain track files are named a1ring2001.wad and s1ct06.dat, so you can replace the original A1-Ring track with it. (And nothing else.)
With GPxPatch, you can replace any track with any custom track and it will read track default setups as well. (Without GPxPatch the game will not load custom setups and additional track data and you will use the original track's values. For example, if you use F1virtual's Bahrain track without GPxPatch, the game will load the original A1-Ring setup for you and for the AI as well.)
My workthread - [www.grandprixgames.org]
Full of classic F1/non-F1 track layouts
My blog about F1 performance analysis - [thef1formbook.wordpress.com]