Set four months after season 3, the members of the 2nd Mass are again fighting the Espheni again. The defense grid is down but that does not mean that the Espheni have given up. Many canon characters open.
The realm of Hades was split into sections. Those who were good were granted Elysium, those who were bad were sent to the Fields of Punishment for endless torture, etc. Now the king of the Underworld has abandoned his kingdom and the walls have crumbled.