Boston’s four at-large city councilors subscribe to an age-old survival philosophy: Separate stalks are easily broken, but tied together they are hard to destroy. Enter challenger Michael Flaherty of South Boston, who is determined to take a weed whacker to this garden party on election day Nov. 8. Incumbents Felix Arroyo, John Connolly, Stephen Murphy, and Ayanna Pressley are running as an informal slate, hoping to block Flaherty.