Chickens dont get colds. Chickens do not sneeze becasue they are cold. Chickens dont gurggle when they are cold. These chickens are SICK. Take them back and sterilize EVERYTHING they have touched. FYI they look older than 4 weeks. You got ripped off.
Probably Infectious Coryz or some other respitory crud. If you want to try to save them-SULMET in the water. Even if they live they will be carriers so if you want to get more chickens you will be risking them as well. You sadly got taken and no at that age unless its like under freezing they dont need light, just protection from wind...a dry place to sleep.