Back 4 Blood's PvP mode, Swarm, was shown in-depth during E3 2021. The 4v4 mode will see players battle it out as the Cleaners versus the Ridden, in a competition to see which team can last longer against the horde.

The matches in Swarm will be best of five rounds, with teams swapping between the Cleaners and the Ridden, with the Cleaners aiming to survive as long as possible and the Ridden trying to take down the humans. Both teams can earn upgrades by getting kills. The upgrade points can be used to upgrade the Ridden horde to take down the cleaners faster or to upgrade the cleaners to survive longer. Upgrades persist between rounds, allowing players to coordinate which upgrades they unlock over a few rounds.

The presentation also showed multiple different Ridden variants: Reeker, Retch, Exploder, Stinger, Hocker, Stalker, Tallboy, Crusher, and Bruiser. The Cleaners can mix and match different attachments on their weapons, across all of the weapons.