Will I need to find accommodations for my fertile duck eggs under my broody hen?

So all of a sudden, one of our chickens decided she wanted to be a mommy and went broody. I felt bad since we no longer had a rooster and none of her eggs were fertile so I put some (should be) fertile Pekin duck eggs under her instead. I put four under her over the course of a couple days, the first one being 8 days ago. I read that hens normally only stay broody for 21 days but I also read that Pekin eggs take 28 days to hatch. Although I know sometimes the hen will decide to stay broody longer, I don't want to rely on that. So will I need to find accommodations for the duck eggs if my hen decides to give up after 21 days? I don't want to spend money on an incubator I'd only be using for a week. Do you have any suggestions or input on how to solve this problem if it does happen? I checked the eggs today with a flashlight too and so far 2 of the 4 are really developed, so I'm pretty sure I can expect at least two hatching. I'm not sure if the other two just weren't fertile for whatever reason or if I just put them under her a few days after the original ones and so they aren't quite as developed yet. Which would again add more time to how long those eggs would need to hatch. So what should I do if my hen decides to not stay on the eggs for 28+ days? Thanks for your help!