For BPs the general rule is 1 square foot of space per foot of snake, so 2x2 would be good for a 4 foot snake.
I don't have any personal experience with Burms, but I don't think that the rule would apply for them. They have a much heavier body than BPs. A 5 foot Burm and a 5 foot BP would probably be pretty different weight wise and would have different housing requirements in terms of size.
Just my 2 cents![]()