Do you have a body shot? The saddle feathers should help a lot with this. When I say "dance" does she approach the other which one wing down and uses her foot to "scratch" that wing?
PS if that is a white rock, then that is a pullet, my 9 week old roo had combes and wattles that were the same size. At 20 weeks his would have been MUCH bigger.
What breed? Does this one "dance" for the other one? At 4 months, most Roos should have bigger combs and wattles. This one looks like a pullet close to laying. If there are no other Roos around and you don't have any crowing, then she is a girl. Also, if there are not roos then a hen will...