Well, technically, MK3 if you want BeoLab 4.
I'm sure MK3 is backwards compatible, so go with that one.
As far as active (BeoLab) speakers, any are approprite. Whatever is best for your space and for you. The usual suspect around here is the BeoLab 3000. But those can be a bit large still and limited to wall placement. A LOT of people like the 4000 which is quite compact. There are a bunch of options...