is 4-6 hours of sleep possible?

It is. I used to do it in my senior year in college. I even went sometimes a whole night no sleep. I read that 4 hours is the minimum for survival.

That being said - it really messed me up. Don't do it. Instead, be very disciplined, cut off anything you do not need, and spend every time you have to do what you need to do. Learn to love your duties and they won't be a burden. This way, you won't destroy yourself.