In response to a question about whether Back 4 Blood could be played offline in solo mode with AI team-mates, developers Turtle Rock replied (opens in new tab), "We're looking into ways we could support offline for the future but you will need an internet connection to play at launch." That's a shame, because both of the Left 4 Dead games support offline play and some players were hoping Back 4 Blood would follow suit. It also won't have split-screen play at launch, with Turtle Rock noting that (opens in new tab), "We'd love to support this in the future but won't be launching with it."
Hopefully both features come in post-launch updates. Turtle Rock did earn some goodwill recently by announcing that DLC will be accessible to all players in a party as long as the leader owns it, as is also the case in games like Vermintide 2. It's a relief to know that DLC won't fragment players.
Back 4 Blood is due to come out on October 12, and it will have a PvP mode that lets a team control the zombies.