Welcome to BYC your measurements of your coop are 4x6=24 square feet
Giving your in the north like I am give them 5 square foot per bird. SO your coop would hold 5 birds. Remember winter is when they will have the least amount of room so I wouldn't try squeezing any more chickens in there. Make sure you have enough perches for the chickens to roost at night also. If you could go bigger on your coop. I would go to 6x6 and keep your 5 or 6 chicken as your count giving them a little extra room to move around in the coop when its snowing and so bitter cold outside. You want to give them room so there is no problems later on from being to crowded.