Usually I go to bed around 22:00 - 23:30, when there is no work.
In my opinion, by sleeping 4 hours a day there is no problem, as long as you can manage your stress level properly and exercise diligently every day.
4 hours is way to short for me. Sleeping around 7 hours is optimal, if I sleep too short or too long I am going to feel it all day and won't be as productive. Recently I started to go bed 1 hour earlier and wake up earlier too so I can do important things in the morning.