United trailed Blyth Spartans 3-0 at half-time – but ralled in the second half with goals from Ezra Aaron and Ellis Perry-Burdett.
However, Spartans added two late goals to seal a 5-2 victory and a place in the semi-finals.
Boston: Zach Biggs, Ellis Perry-Burdett, Josh Doran, Rueben Fogg, Finley Ging, Taylor Jaine, Joe Wilkinson (Sam Finch 70), Kris Zaluzinskis, JJ Ward (Ezra Aaron 46), Lucas Turrell (Charlie Brooks 46), Luke Horne. Subs (not used): Josh Panton, Nathan Bowen.
Referee: Trevor Basker.