Yes, Back 4 Blood crossplay is a thing. The co op zombie shooter can be played across platforms. You and your friends can indeed team up to slay the undead hordes no matter the platform you are playing on.
On a side note: If zombie shooters are your thing, DayZ [PC, PS4, PS5, Xbox & Switch] which was started as unofficial mod for ARMA 2 might also interest you.
Coming back to cross platform support in Back 4 Blood, joining up with friends isn’t as straightforward as you'd think it would be. So let us tell you how to exploit this feature.