There are 32 - 5 stars characters. There are 78 - 4 stars characters.
Why only 21 of 78 4 stars champions characters have 5 stars covers rewards?
The 5* cover rewards were started in mid or Nov 2017. Initially, 3 feeders were updated and a feeder for 5* were “announced” monthly. The schedule changed for a while but we end up with 21 feeders anyway.