Possible cyst/tumor on 4 week old chick???
It's possible that it could be a leaking air sac from an injury to the neck, back, or chest. Are you able to identify the chick's crop, to make sure that isn't what you are seeing? Leaking air sacs can be punctured with a 22 gauge to 18 gauge needle inserted just under the skin, and the air can be pushed out. It can come back again, and need to be re-done. Personally, if the chick is not having any problems breathing or the bulge is getting larger, I would leave it alone. Keep an eye one the chick's crop to make sure that it is normal like the other chicks.