When you learn to how to juggle 4 balls, it’s a bit different than juggling 3 balls. The reason is because the pattern is not a cascade. When you juggle 4 balls you are juggling 2 balls in each hand.
So the key here is that you learn how to juggle 2 balls in one hand comfortably first. Once you get that down all you have to do is learn how to do it at the same time.
Go buy my recommended juggling balls.
How to Juggle 4 Balls Other Ways
The video above shows the typical pattern for juggling 4 balls. There are other ways to juggle 4 balls. One other method is the 4 ball shower. This is when you throw the balls in a circle pattern. It is much more difficult because you are not juggling 2 balls in each hand anymore.