Are there any 4-seater cars that don't have a rear window?

No - if you skim through https://googledrive.com/host/0B-Q7WnzcPvLtLVVoM3otTVlnOFE/all.htm you can see that basically anything with 4 seats is going to have a back window. My best experience with getaway vehicles so far have been the Jackal (inexpensive, easily stolen) or the Exemplar (200k or so, performs well.) Super cars don't appear to have back windows, or they're very small but I've managed to shoot and kill other players in them during a chase.

I'd suggest stealing a jackal and tweaking it out a bit to try, since you can get the car for 'free'.

Just to note, my experience with regular vans and cube vans is that the back doors or cargo door will stay open during travel, especially if people are shooting at pursuers.

Sandking XL and buy the painted double roll bar (chassis). bullets will no longer enter from the rear window. shooting out of the back from the truck will break the glass. however it will still block shots from entering even with the window broken. works on the current 360 version. don't know about ne... current gen consoles.