Hens come in different sizes. Eggs come in different sizes. Some smaller bantams can barely cover 4 regular sized hen's eggs. I hate to think how many bantam eggs some of the larger full-sized hens could cover.
This is another one of those things where there is no magic number, you have to use your judgment. You want the hen to be able to cover all the eggs she has. If she can't, different ones get pushed out and can cool off, then get pushed back under her while another cools off. You usually don't get good hatches when this happens.
Is she isolated from the flock or hatching with the flock? If she is hatching with the flock it's possible another hen or two could lay an egg in her nest. I like to leave enough room to accommodate that possibility. I do mark the eggs and remove any new ones daily.
If the four eggs have been under her any length of time you do not want to add any more. That would create a staggered hatch, which means some would hatch earlier than others so the later eggs will almost certainly be abandoned. You do not want a staggered hatch, they are way too stressful and usually a big disappointment.
When I was a kid we had a broody hen hide a nest and show up with 18 chicks. I never did find her nest so I don't know how many eggs she started with, she must have laid a fairly small egg for her size. I normally give a hen 12 eggs the size she lays when I have a broody hen but I have had some full-sized hens that could only cover 10. How many a hen can cover is a judgment call.
