Firstly, do you have a cockerel/rooster that has been mating her? The eggs will not hatch if they have not been fertilised. Apologies if that sounds like an obvious question but some people don't seem to realise that it takes two to tango for hens as well as us humans.
Secondly, has she been sitting in the nest, incubating the eggs day and night for 4 weeks or was she a bit hit and miss at the beginning or was she just starting to assemble her clutch of eggs 4 weeks ago.
Thirdly, how many eggs does she have? If she has too many and can't cover them all, all the time, then that may have caused some of them to chill and then the hens rotates the eggs and some different ones get chilled.... that can lead to a late hatch (but not a week late) or the foetus dying.
Could the broody have been trapped off the nest for a long period of time or got back onto the wrong nest by mistake after a broody break and the eggs got cold?
Have you candled the eggs at any point? If so, was there any development when you checked?
If she has been sitting tight for 4 weeks then something has gone wrong, however I would listen for any sound from the eggs. If no sound, float them in a bowl of warm water and look for movement and if none, select one and carefully crack it outside somewhere where it can be buried and the smell won't be a problem and do it at arms length..... beware they can explode! Once you break it, it should be obvious if there has been any development or not,,,, ie the egg was fertile and there was some development but it died at some stage for some reason or the egg was infertile and was never going to develop.
Sad for the hen and you might have to break her of her broodiness even after you take the eggs away.....search broody buster cage for more info on that..... as some hens will not give up until they get chicks and brooding for too long can damage their health.
Good luck and do be careful as those eggs can be pretty explosive and stinky.