It is 4+1
"Magazine: tubular, four-round capacity , three shot adapter provided (2+1)"
Full article here
americanrifleman.org/articles/2012/7/31/the-beretta-a400-xtreme-shotgun/

Taylor, 4 rounds in the mag w/plug removed.

Taylor, this Beretta Shotgun has a 4 round tube and is a 5 Shot model with one in the chamber. 3-1/2" Chamber (Accepts 2-3/4", 3" and 3-1/2" Shot Shells). If you need even more capacity, tube extensions are here : midwestgunworks.com/page/mgwi/prod/brtext -- Cheers and stay safe!

Extensions available here
https://graceshootingparts.com/magazine-extensions/benelli-magazine-extensions/briley-magazine-extension