I taught myself in junior high to juggle three balls with two hands and two balls with one hand. It's not a huge accomplishment but what amazes me is that I can go years without trying it and when an opportunity comes up I can just do it again, within just a couple of tries. Those neuronal connections just never go away.
Learn to juggle two balls with the other hand and you are half way to 4 (which is two in each hand out of sync)