Goals from midfielder Lewis McKue and strike duo Juris Kormis and Karolis Andriulionis sealed only the Pilgrims’ second league success of the season.
United trailed 1-0 at the break, but roared back strongly after the half-time to record a deserved win.
Boston (4-4-2): Tom Benton; Niall Delahoy, Jordan Nuttell, Joe Cain, Jordan Sowerby; Leon Squire, Joe Briers, Aaron Eyett, Lewis McKue; Karolis Andriulionis, Juris Kormis.