If Johnny only sleeps 4 hours every 4 days he is going to have problems, both mental and physical. We're supposed to sleep 8 hours per day, which would be 32 hours every 4 days.
As far as what percentage Johnny is sleeping, we can't answer that without more information. Do you mean what percentage of what a person is supposed to sleep? Do you mean what percentage of the entire day, on average, Johnny is sleeping?