I agree with what has been said. If the nesting boxes are only four inches in height, then chickens are not going to be fitting in there and laying eggs. Keep in mind that a female chicken goes into a slanted body position when she is actually laying the egg, half-sitting and half-standing, so the top of the nesting box needs to be taller than her head.
I use straw in my nesting boxes because it provides better airflow, but personally I think hay would be a good option too, even though it is more expensive. But keep in mind that some hens just do not want to lay their eggs on anything comfortable apparently, and if they do not want to then let them. The hens in one of my flocks used to completely pull their straw out of their nesting box when they were pullets, and they still to this day make nests in the straw but they leave the bottom completely bare of straw so they are sitting on cold, hard wood. Sometimes I will never understand chickens but they are funny with their preferences.
Post pictures of them here so we can identify them. However, you should automatically be able to tell the difference of a bantam versus a large fowl. A large fowl chicken can be any chicken that is the "normal" size that you first think about the "average" chicken being, and anything larger than that. A bantam will be typically 1/2, 1/3 or 1/4 smaller than that.