OK, this is such a messy route, and it's further confused by https://www.mbta.com/schedules/4/line?schedule_direction%5Bdirection_id%5D=0&schedule_direction%5Bvariant%5D=4-3-0 being inaccurate. If you look on the printed schedule https://cdn.mbta.com/sites/default/files/media/route_pdfs/batch_6374/4-S1-P3.pdf you can see the real route. I also got the name backwards, which I'll correct in the next changeset.