So three balls just isn't challenging enough anymore? Everybody who sees you juggle asks "Can you do four?" If you want to answer "of course", read on.
Beginners often try to juggle 4 balls using the same pattern as 3, throwing from right to left and left to right at the same height and a steady pace. Assuming you have an even number of hands and an even number of balls, the cascade simply won't work. Your hands would have to throw at the same time for the balls to be thrown at a steady rate, and that's not a cascade. The most common way to juggle 4 balls is to juggle 2 in each hand, alternating right and left hand throws. This pattern is called the asynchronous fountain. Learning to juggle 4 balls this way isn't too difficult once you can juggle 3 balls.
Two In One Hand
First you need to learn to juggle 2 balls in one hand. It's pretty simple, but it will take a little practice, especially in your non-dominant hand. Hold two balls in one hand, and throw one of them up and to the outside, a little higher than you usually juggle 3 balls. If you're using your right hand, try for a throw that would come down a few inches to the right of where it was thrown. When that ball peaks, throw the second one the same way. Catch the first one and when the second ball peaks, throw again. Just keep throwing from the inside of the pattern and catching to the outside. Once you're starting to make progress, learn to juggle two in your other hand.
The most common difficulty in juggling 2 balls in one hand is turning. While juggling two balls in your left hand you may be tempted to pivot to the right. When that happens, concentrate on bringing the balls further toward the center (right in front of your naval) before each throw, and throwing them further to the outside of the pattern. Your right hand will be making clockwise circles as you throw and catch, and your left hand will be making counter-clockwise circles.
Four Ball Asynchronous Fountain
With two balls in each hand, make a throw from one hand and immediately make a throw from the other hand (before the first ball peaks.) Then it's a matter of juggling 2 in each hand. Stay relaxed, and keep your eyes on the top of the pattern with the balls peak. If you've had enough practice with juggling 2 in each hand separately, you'll find you get the feel of 4 ball juggling fairly quickly. If it seems hopeless, spend a little more time with just two and then try again.
Other Four Ball Patterns
When you're getting the hang of four ball juggling, consider adding tricks. And of course you'll want to learn the five ball cascade!