Honestly, I'm not sure 20 seconds every 4 hours is enough for a cham of that age. What size cage is he in and does he have real or fake plants and vines? Does your guy have a dripper for hydration between mistings? Also, what are your humidity levels? Humidity plays a factor in keeping your cham hydrated as well, so if its not appropriate, that could cause issues as well.
Anytime I am worried that my chams aren't drinking enough I get my small syringe, some water and a towel, then I sit down with my cham and slowly drip water over his/her nose until eventually...and I mean you HAVE to be patient...they will drink. Once they get a taste, if they are thirsty they will drink. If they really won't drink after tasting it then I know they have had enough from my regular mistings and their 'big dripper'. I just feel more comfortable seeing them drink at least once a day.
If you suspect your cham is dehydrated, he probably is, or close to it. Dehydration can happen quickly and is the catalyst to further, more serious health issues. Try putting him on a plant in your shower but have the spray bounce off the wall and not hit him directly. Water should be warm and it will create a mist which will hydrate him. 10 - 15 minutes will help immensely. If this is not feasible then drip water over his nose, as above or manually mist his cage with a spray bottle.
Above all else, if you are worried or have a concern, take him to the vet. This is the best possible solution to any questions you may have.
Good luck!