I have an almost 2yr old and an almost 4 yr old and I am almost bald!

I do have the benefit of hindsight in that I have older boys so clearly I survived (not to mention so did they) but I think I have mentally blocked out that part of my life.
My DS can be to put it mildly a total little ****wit! He terrorizes his sister from the moment she wakes till the moment she goes back to bed.
He can honestly spend more time in the toilet (our timeout place) than out of it somedays. He runs off screaming NOOOOOOOOOOOOOOO whenever he gets sent there but to his credit he does go. He knows the consequences are so much worse if he doesnt.

I have found getting outside with him for at least half an hour in the afternoon to really run him round has helped a little.
He has so much pent up energy and is really bored so this is a massive release for him.

We saw a behavioral physiologist with our DS9 and this is what he reccomended to us, it is from the PPP parenting.
Just really slam down on him (not literally of course ) but show him that you are the boss and you will not tolerate this behavior.

Tell him to stop what he is doing and tell him what you do want instead.
Repeat your instruction with "or you will go to time out".
Straight to time out, (2 minutes for under 5's) you may have to hold door closed
Let him out and dont talk about the bad behaviour but try to "catch" good behavior as soon as possible and over praise.

I know its hard when all you want to do is strangle them or leave them in the toilet all bloody day but it does work more than not in our house.

He will fight (like a demon child) and push back but you cant give in. It is called the bursting effect and apparently everything from monkeys to dogs to kids will act out and fight back when you start coming down on them but you have to stay strong and they will soon learn as long as you are very consistent that it is not worth it.
If you do do give in you have moved the line he knows he has to get to before you will cave. So it will become worse

Huge hugs, it is really hard and draining but remember the teenage years are even better