I tried and failed to learn to juggle three balls many times, I've just got terrible coordination. But one day I stood over a bed and just threw them in the air and listened to the rhythm of the "thuds" as the missed balls hit the mattress. As soon as I'd got that down it was like a switch clicked and my hands knew "when" to be ready for the catch, rather than trying to follow the balls to catch them. I never managed four, so mileage may vary with this technique, but it was a very surprising lightbulb moment.

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.