[Lisa~]

Aoife is right on track with her answer. Try to figure out how to lunge without hurting your knees. Let that be your focus right now. Don't decide fully about extending this stage or not until you're done with planned time-frame. And if your knees are still hurting at that point, then move on to stage 4 and hopefully the change of exercise choices will give them a break. You'll come back to these movements soon enough.

This is alternating periodization, that's why you come back to the same exercise scheme with a lower rep range. Repeating the stage 3 and stage 4 series would make more sense to me than prolonging stage 4 by itself. So you've got options.