No problem and yea ahah guess thats how it works lol :D around 8 or 9 is comfortable it really depends on the size of chicken bigger chicken more eggs smaller chicken less eggs :) , if u sent me a pic i could probably give you a better answer :)
And yes 1 hen will make all the hens eggs fertile the best ratio is about 8 or 9 hens to a rooster, you could have more hens but the chances are the fertility rate would be lower, so it should be fine in with the four u already have, and the fertility rate should be higher :):)