For specific suggestions I'd need to know what incubator you have. A photo or preferably a link so I could find it online. And what variety of ducks? Some duck eggs have a longer incubation period than others. Muscovy are five weeks.
You can generally stop the turning a week before hatch. You've already achieved the benefits of turning by then. We usually wait until lockdown to stop turning and increase the humidity. It's simpler to do it all at the same time. Still you'd need to stop turning a couple of days earlier than you'd want to with this relaxed method. I'd consider that an option.
The turning is not the only thing I'd worry about in a staggered hatch. When they hatch the eggs will have some liquids and such in them other than the ducklings. That can make a slimy mess. Also, shortly after they hatch the ducklings can poop. That mess inside the humid incubator can cause bacterial growth. The incubator can start to stink and the bacteria can kill the unhatched duckling in the shell. This does not happen every time but the risk is real. It happened to me once, that stink was horrible.
There are reasons people that regularly do staggered hatches use one as an incubator and one as a hatcher. It makes life a lot simpler. I was planning on making the same recommendation Blue did, be sure the egg is fertile before buying another incubator unless you plan to use it in the future.
Another option would be to make a cage out of hardware cloth or maybe use one of those mesh baskets berries sometimes come in and invert that over the late egg. Keep the other ducklings from crawling on that egg and sliming it or pooping on it. Then after the first hatch is over thoroughly clean the incubator before you lockdown that last egg. You should have time to clean before lockdown. Sterilize it well and you should be fine.
Good luck with it, hope it is fertile and develops.