I had a friend who had absolutely terrible coordination.

I would bet against him being able to learn three balls.

But he was very dedicated. Long story short. After many years he could comfortably juggle 6 (six!) balls.

It looked wobbly, he still looked like a person without coordination, but the balls somehow stayed in the air.

Four is just two in each hand. Learning to do that one hand at a time is a nice addition to a three ball cascade. You hold the third in your other hand, and it's a good source of jokes.

You can do four-out-of-five, which is a five ball cascade with a missing beat (just as how you learned two-out-of-three while learning initially). Unfortunately it's very hard and still doesn't impress people.