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.