In the season 3 finale, Poet and Pierces bandages are taken off and doctor Nathan tells them that their skin won't go back to normal for 6 months to a year. In the season 4 premiere, which takes place two weeks later, their skin is fine.
In the gym, Guillaume Tarrant is wearing bright hospital slippers which keep disappearing and reappearing.
The gun Adebisi has, given to him by Hughes, is not the same gun he had in the season three finale. The gun in the finale appeared to be either a prop gun or a .22 starter pistol. The one he has here is a compact Glock.
(At 11.29) Ryan O'Reily cuts his hand with a butcher knife in the prison kitchen. But a tube inserted into his sleeve to simulate the gushing of blood is visible.
The Glocks being carried by the SORT team are clearly plastic prop guns, they don't even have a bore.