Understanding sets

[dandesix_PSN]

I thought I understood sets and my lineup until now…while currently trying to insert guys into my lineup, I need a season 4 eligible lineup, which according to the upper right of the screen needs to consist of core/set 3/set 4 players, as well as the use of one wild card from any set. My lineup has all core and set 3 players in it, with only one set 4 player which the game is listing as my wildcard. When I try To insert a second set 4 it still wants to only allow one set 4 as the wildcard…why can you only have 1 set 4 card in your lineup?

[Dolenz_PSN]

If you have an open Squad spot choose to create a New S4 Squad.
If you have all your squad spots full then delete one and create a new S4 Squad.

The game seems to have issues sometimes converting previous season squads to the newest season by simply replacing players.