The result saw 'Side exit the Trophy in the Second Round of the competition. The win was St Margaretsbury's second win over the Sky Blues in three days, following their 2-0 win in the league on Saturday at Cricklefields.
'Side suffered a poor start to the game going down to 10 men after only 16 minutes when Ryan Karrer was sent-off. With no substitute keeper on the bench Harry Matthews was forced to go into goal for the rest of the game.
The hosts would soon take the lead through Chancey Dash on 18 minutes. However Michael Walter's men, against the odds, would go into half-time level through a great strike from Jedidiah Brown on 32 minutes.
St Margaretsbury's man-advantage would tell though in the second-half as Dash would score twice more, bringing up his hat-trick with Connor Scully and Luke Provenzano also getting on the score-sheet.
Brown with the score at 3-1 did grab his second of the game to make it 3-2, with 'Side also having a perfectly good goal ruled out moments before St Margaretsbury's fourth goal.
John Barker's men will now face Takeley or Stanway Rovers away in the next round. 'Side are next action against Wodson Park (Spartan South League) in the FA Vase on Saturday at Cricklefields. Kick-off for that match is at 3pm.
'Side: Ryan Karrer, Garjay Thompson, Aaron Hunwicks (Hameed Lawal), Harry Matthews, Euan Taylor-Reid, Danny Laurent (Romar Johnson), Mattheus Santos, Callum Matthews, Terry Anim (Jacob Dingli), Manny Brown, Jedidiah Brown
Sub Not Used: Thomas Ngegba