Despite it being cancelled, plot developments had been made for Season 4 and this would have been the basic storyline of the season:
Episode 1: The gang bury Robin's body and accept Archer as their new leader
Episode 2: Tuck leaves to do more traveling
Episode 3: Much realizes Tuck is heading to trouble and leaves to warn him
Episode 4: Prince John comes back and Archer tries to make him compromise to not kill King Richard
Episode 5: Prince John escapes, and Blamire (who survived the castle explosion) finds Archer and the gang and ultimately kills Kate.
Episode 6: Tuck and Much return, as does King Richard. Much and Little John tell him everything.
Episode 7: Prince John brings an army to kill the 4 remaining outlaws, and Richard tries to settle it by having a personal fight between the King and Prince themselves
Episode 8: Prince John makes Blamire the new Sheriff and has him rebuild Nottingham Castle
Episode 9: Much is kidnapped in the castle's dungeons, and Prince John interrogates him about Archer's plans
Episode 10: Little John thinks Robin and Allan's ghosts are telling him something
Episode 11: Prince John realizes he is alone and tries to kill Richard and the gang himself
Episode 12: Prince John successfully kills King Richard
Episode 13: The gang are forced to surrender when they realize killing Prince John would put England in trouble. They almost give in their lives, but two old gang members return from the Holy Land (presumably Will and Djaq).