4 is for 4wd but RB is just random kinda like FXO, the F is for fwd XRT, R for rwd etc. i'll take some whacks as to what it can stand for; rally based 4wd ..ya thats all i got lol
Actually it was "RainBow 4 You" originally, but they had to cut the U because of the 3-letter names we had back then