The result means that the South Africans have already won the series 3-0 with one game drawn before the final encounter this afternoon.
After Holly Cram missed an early opportunity for the Scots, Coetzee beat Nicki Cochrane with a fierce reverse stick shot to put South Africa ahead. Three minutes later, however, Scotland were level when Cram won a set piece, the ball being switched left by Becky Merchant and Ailsa Wyllie sweeping the ball home.
Coetzee, though, was on the mark again at South Africa’s first penalty corner of the match when she sent an unstoppable drag flick into the Scottish net.
The second half was largely a midfield stalemate, but in 58 minutes Coetzee released a fierce reverse stick shot past Cochrane – who made a number of good saves – for 3-1. Kate Woods then Ceotzee, again, made it five, before the battling Scots pulled a goal back when a hit by Merchant at a penalty corner was brilliantly deflected into the net by captain Linda Clement.