I had 3 female robos in a bin cage together for over 2 years. They were lovely together. Just make sure they have as much space as you can give them and as much to do as possible. Mine used to go into a large playpen together regularly too.
My 2 male robos lasted about 1 year together and then decided they didn't like each other any more.
I had to split them up.
Good luck. I love the idea of keeping 4 together.