A 2x4 is to small for 4 piggies, you need a minimum size of a 2x5 atleast. Girls dont need as much space as boys do but they still need a bigger cage pronto. Its a very good job on the cage itself, but try and make it bigger as soon as possible.
We're just trying to make constructive criticism. I would suggest listening to some suggestions given. If you don't have enough room to expand the base, you can always expand the loft, it's quite easy.