How many hammers is the Henge on these settings?
Note that when slaving, the current turn's production (including chops) doesn't count towards the total or affect how much population you need to slave it. So if your chops finish the same turn that you plan to slave, they won't reduce the amount of population you need to slave it.
So finishing the chops on turn 4 and slaving the same turn doesn't make much sense unless you're intentionally going for the gold overflow? Might be better to slave on turn 5 (so the chops have already been added to the hammer count before slaving) or maybe move 2 workers onto the same forest and get 1 chop on turn 3, so you can (maybe) still slave on turn 4.
I agree that anything which delays completion is a bad idea though. Missing out by one turn would be disappointing.