I noticed that there is a white police4 (undercover stanier) that sometimes responds to pursuits. Is there any way for the player to get this in game with ragehook? I've scrolled through the options in the LSPDFR car-selection page as well as spawned police4's multiple times through the RH console, but I only see black, gray, blue, and red variants.

You can use the trainer to change the color, or just spam a request for State Patrol backup. Eventually one of them will arrive in the white slick-top and you can just 'borrow' it.

Most likely the reason you can't get a white one through the selection menu is that the color is not naturally available in the game data for that car; the "carcols" and "carvariations" files that control a lot of the initial content are of a still uncracked filetype.

As was mentioned, using the State backup options or using a trainer to manually change the color are your best bet.