A less advanced prototype of Data, found on Kolarus III and used by Shinzon to lure the Enterprise-E into a trap.
B-4 was an earlier android built by Noonien Soong, similar in appearance to Data but far less capable. In Star Trek: Nemesis the Enterprise-E found him in pieces on Kolarus III and reassembled him. Data tried to share his memories with B-4, but the prototype could not make use of most of them.
Shinzon had planted B-4 to infiltrate the ship and steal tactical information. Data took B-4's place to rescue Picard from the Scimitar. After Data's sacrifice, the crew hoped that something of him might survive in B-4, and B-4 was seen beginning to recall a song Data had sung.