That is correct. 4 RBs is too much and 4 WRs is not enough. If the intention is to move Sermon to the PS, then that's fine.
Also, having the ability to move a player up 3 times a season should not really be considered depth when you are only carrying 4 WRs. Cain looked good in the preseason, but he is 26 with 9 receptions in the NFL. Allen and Covey are special teams players. So yes, I would rather have Reagor on the roster.