due to the size and thrashing of the rbp when i would walk past the tank, they did crack my tank at the seem. I had to drain the water and re-seem the tank. 4 in a 30 will last for alittle while. (less than 6 months) the ideal situation for you would be a 55 with TWO 2 TWO thats it 2 rbp's. Honestly I found that when I had 1 it ate when it wanted to, and there was no competion for food, so there was no real rush for it to eat. So I understand that aspect of a RBP, so two in a 55 should work for awhile.
Also you can not add any other RBP once they continue to grow. The stronger of the two will eat the other one.
as far as food. the ones i had for 16 years I feed anything and everything. Live food, gold-fish, lizards, blue-gill, crayfish, worms (they love worms). When they were small I would throw in a frozen piece of brine shrimp and they would learn to eat bites off of the floating frozen mass. small worms work to when they are small.