Quick Look:
A Four Ball Fountain means that both hands are juggling two-in-one-hand, and the balls never leave either hand. In other words, the balls in your RH will always be in your RH; the balls in your LH will always be in your LH. When juggling this pattern, there are two choices that you have to make:
First, you need to decide how you're going to juggle two-in-one-hand. Fountains are much easier for bounce juggling, so that’s what I recommend. Note that both of your hands should be doing the same thing! Don’t do columns in one hand and a fountain in the other. (I suppose you could try, but it would be unnecessarily difficult.)
Next, decide if you want to juggle synchronously or asynchronously. Synchronous means that your LH and RH throw at the same time. Asynchronous means that one hand throws slightly before the other. Asynchronous is typically easier for people to learn, and it also looks cooler because more things are happening. (Weirdly, asynchronous was easier for me to learn for bounce juggling, but synchronous was easier for toss juggling.) Learn what's easiest to you first. To start asynchronously, start juggling two-in-one-hand a split second after the other hand. You should learn both synchronous and asynchronous patterns!
Practicing a Four Ball Fountain is very straightforward. First, try to run the pattern as long as you can. If you find yourself struggling, work on two-in-one-hand in your desired style (i.e., Fountains or Columns) with one hand at a time. Practice juggling two-in-one-hand without looking directly at the pattern; keep it in your peripheral vision. You need to do this because when you’re juggling four balls you can’t fully direct your attention to either hand. Once you’re comfortable doing two-in-one-hand with both hands, move back up to Four Ball Fountain/Columns. Repeat this process as necessary.