BB-4 was an astromech droid from the BB-series with tan plating who served the Resistance. The droid's sensor was partially exposed on its head, a trait it shared with 2BB-2, another BB unit working for the Resistance. BB-4's body also displayed the Resistance crest.
This droid navigated and performed maintenance by rolling through the hangar bays of Resistance outposts and capital ships, and it also plugged into the sockets of T-70 X-wing starfighters.