Although Back 4 Blood split-screen is currently not a feature of the game, and will not be available on release, there is good reason to believe that it may be included down the line.
At the Back 4 Blood showcase at E3 back in June 2021, the developers of the game, Turtle Rock Studios, held a Q&A session. The question will Back 4 Blood support split-screen? was asked by one of the attendees of the event, and answered by executive producer Matt O’Driscoll.
Mr. O’Driscoll said that Back 4 Blood wouldn’t feature a split-screen option at launch, but that they would love to support the feature if it is heavily wanted by the community.
We already know that the feature is indeed heavily wanted by the community, so if the game continues to be as successful as the beta suggests it will be, and Turtle Rock manage to get the time to implement such a feature, chances are good that we will see split-screen in Back 4 Blood down the line.
You can see the full E3 showcase of Back 4 Blood and the Q&A in the video below (around 5hrs 40 mins in).