> you either have to decrease the time between catching a ball and throwing it again or increase the time a ball is in the air

I think you might be thinking of 5 ball juggling.

4 ball juggling (or at least it's most common variant, "The Fountain" [0]) is fascinating because it's really juggling two balls in each hand in a way that makes it appear similar to the standard cascade. Though this may sound "less hard" than what people initially imagine, it's a very different feeling than all the basics you learn using only 3 balls.

0. https://en.wikipedia.org/wiki/Fountain_(juggling)

In my experience people can’t even tell how many balls you’re juggling after 3. 4, 5, 6, 7 all get “how many balls is that”, silly stuff like 3 ball factory blows more minds than 5 ball site swaps

Also, with a three ball pattern, most of the time there's only a single ball in the air. With four, there are almost always two. The odds of a mid-air collision increase significantly, and go up as the number of balls increases.

That also forces you to have much more consistent throws which, as you note, gets harder because you also have to throw higher which scales up any error in the force you're applying.