My 4 year old has enlarge adenoids he snores a lot foams at the mouth an wakes up coughing every night. I'm starting to worry about this. What could be the cause of this?

I am not in the medical profession, but foaming (assuming clear or white and not red) is air added to body fluids. Your child has enough swelling that his condition is blocking his airway. This also explains the snoring (partially blocked airway). Bu all means take your child to the doctor right away, before the airway is totally blocked. They can check for allergies, or clear up the infection causing the blockage.