Here are a few 4 ball lessons...

 

Juggling Stein Tutorial - 4 Balls:

The concept of juggling 4 balls is very simple. You juggle 2 balls with one hand... with both hands! I disagree with this person when he says that 'syncronous' is harder. I believe it is easier to learn 'syncronous' but you'll probably find yourself juggling 'asyncronous' a lot more when you learn how to.