I think that size would work well. Most breeders use cages that are 2' x2' for each lion so I think a cage that is twice as big would work fine for twice the rabbits. Especially if you plan on letting them out to exercis every day.
Your right about their size. Lionheads commonly only get to be 3lbs. Mine is just over 4lbs and she is way bigger than any other lion I have seen.