I would say it depends on what he does the other 8-9 hours a day. If he's sitting at a desk in school/ chair to eat/ car to get to & from... then the only exercise he's getting is in his fingers & his mind.
If on the other hand he's super active during those 8 hours (aka not in school) but climbing trees, riding bikes, swimming, playing with friends, saving the world, playing sports or music, what have you... then it doesn't seem outlandish. But 4 hours a day of electronic time IS 1/3 of his day. If he's sitting on his bum for school then 91.6 percent of his day is spent not moving.
So how is his balance?