All together, the scene "And magic makes it all complete" has:
The mane 6
The CMC
Zecora
Snips and Snails
Mr. And Mrs. Cake
Mayor Mare
Big Mac and Granny Smith
Photo Finish
I thought that those ponies were probably the "most important" ponies (not counting Celestia and Luna of course), which made me think that maybe those listed would have more major roles in S4.