My Baby's Size
The BumGenius 4.0 diaper comes with two inserts to use with it- a small and large insert. It's a pocket diaper so the inserts go inside the pocket. You change the entire diaper each time, just like a disposable.
The Flip system is a shell that you change the inserts out when dirty, assuming the cover is still clean.
You can use the flip inserts in them. A few weeks ago when cottonbabies was having their seconds sale I bought a few BGs 4.0 that didn't come with inserts and I sometimes use my flip inserts with them, I've had no problems.
ETA: FYI you can't use the BG inserts with the flips because of they are 2 sided microfiber.