You must unlock the Tier 4 map, "Riot". To do so, you must unlock each tier in succession. Tier 1 is available by default, and you must complete 50 rounds in any combination in those maps to unlock Tier 2. To unlock Tier 3, complete 75 rounds in the Tier 2 maps. To unlock Tier 4, complete 100 rounds in the Tier 3 maps. Tier 4 only has one map; "Riot". Complete round 10 in Riot and a DNA Bomb will drop. It will infect your enemies and turn them into zombies for round 11. Complete round 11 to view a cinematic that shows a large number of zombies overrunning the area.