Coops generally have roosts for sleeping and nests for laying eggs. I think you are talking about what people here call a nest. You only need one nest for four birds, about 12" on all sides. You can also use most any container you have around, and it can be open on the front or on top. The roost should have some width to it for their comfort. The wide side of a 2x4 or a thick branch work well. They like about 10" of roost space for each chicken. You don't want them sleeping in the nest because they will poop in their sleep, resulting in poopy eggs. Make sure the troost is higher than the nest, as they prefer to sleep in the highest spot they can.
Good luck, and enjoy your chickens!