Why 4am? If your teen must arise at that hour for some reason, the bedtime revolves around stage of growth/development to get adequate rest. A 14yo (growth spirt) may need 10 hrs while a 16 yo may get by on 8.Any less is trouble. If the kid awakens at 4 & can't get back to desired sleep, you should discuss the issue with your dr. Falling asleep during routein daily activities is a sign of inadequate sleep.
7-8 p.m. A teen should get no less than 8 hours of sleep, so it falls on/around 7-8 P.M.