BB-4 was a tan-plated BB-series astromech droid in the service of the Resistance. Similarly to 2BB-2, another BB unit in the service of the Resistance, the droid's sensor was not fully encased in its head. BB-4 also had a Resistance crest on its body. It rolled about the Resistance's hangar bays of outposts and capital ships and was also used in the sockets of T-70 X-wing fighters for navigation and maintenance.