In all honesty... 4 slot usually isn't going to have nothing useful in it. You can easily go along with 3 slots if you want.
Blue mod (car spawner, ammo trunk or mobile radar), purple mod (explosives or high-burn fuel) and orange mod is all which are useful (for example nitro boost or brick). 4th is usually waste - you put something on it just to add flavor (expection is muffler, red mod but meh... it's mediocre)