Piggies are really sociable creatures. I have an indoor cage, 144cmX76cm, I currently have 2 in this. Although I am currently looking into adopting another female, I will add a second floor before she arrives. The more room you can give them to run around in the better, they get crazy giddy spells where they run and popcorn all over. With plenty of hiding places you could easily have 4 piggies in a cage that size (5 foot long).
They'll be spoilt for room.
A.x