Have you searched for a hidden nest? That’s the most common reason other than the molt for them to suddenly “stop laying”.
Is something getting the eggs? Many critters like skunks, possums, and raccoons normally leave evidence so you should know if it were them but other critters can take them without leaving any traces.
A snake leaves no trace but a snake generally eats some eggs then stays away for a few days while it digests them, then it comes back. That does not sound like a snake.
A canine will swallow an egg and leave no trace and can eat a bunch at a time. A fox or coyote would probably be more interested in the hens so it is probably not one of them. A dog however may eat eggs and not bother the hens at all. Do you have a pet dog that has access to the nests? It could be a neighbor’s dog.
A human will take eggs and leave no trace. Creepy thought, isn’t it?
It’s possible you hens have turned into egg eaters but they normally leave evidence, either some shells or a soggy mess in the nest.
Some diseases can cause them to stop laying but that’s usually a little more gradual and you will often get some funny eggs before they totally stop. Besides you should be seeing some symptoms if they are not healthy.
Sometimes a sudden change or some kind of stress will stop them from laying but usually this would not be all four and some usually come back to laying by two weeks. This stress could be anything from running out of water for an extended time, a change to housing or ways you manage them, a predator scare, a disruption to the pecking order, or just anything out of the ordinary.
With the days shortening they may stop laying without molting. Some hens are better winter layers than others. Again though this should be more gradual and probably not all of them.
It’s also possible they are molting, strange as that sounds. Some molt a lot more slowly than others and they usually start around the head area. You often cannot tell they are molting just by looking at them but you might see a few extra feathers laying around. Again, this should have been more gradual, not all of a sudden, and given their age, probably not all of them.
Any if these things are possible but most of them should be more gradual, not all of a sudden for all four. I lean toward a hidden nest or a dog or human since you didn’t mention evidence of egg eating. But who can know from across the internet?
So what can you do? If they are tightly confined look for a hidden nest. Sometimes they will surprise you how well they can hide one. If they are roaming and your coop or coop and run are big enough, lock them up for a day or two to see if you get any eggs. That may mean they are hiding a nest or you locked something out that was getting the eggs but at least you know they are laying. Put a mark on an egg or two and leave it down there. If it disappears you know something is getting them.
Good luck with this. This type of thing can often be quite challenging.
