I have several pens. Some of my pens that have near 20 birds ( young chicks rangin from 2 to 3 month oles ) I have 3 gallon waterers. IF there is one rule I like to stick to, is change your water every day. No excuses. Any size waterer will get chicken pooh in it. It will get a slimmy feeling to the touch. that is not okay. And that is how chickens or any critter gets sick. With 3 or 4 birds, I would get a 2 gallon waterer. And change it every day. Whether you think it needs it or not. Trust me, my birds LOVE the nice fresh water. Their healthy as horses. Yours will too.