WAG here,
You probably have a 20-30 amp 2 pole (220) and 4 15-20 amp (110) circuits. If you don’t need 220 you could install 2 single 110 breakers in the slots for extra circuits.
Either way I wouldn’t do it, buy the 10 slot don’t mess with swapping breakers or running 2 circuits off of one as it could and probably will backfire on you in a unpleasant way.
