There are actually 5 frames of prejump in KI.
It used to be some characters were less than 5, but in Season 3 they standardized it so every character is 5 frames of unthrowable prejump now. When you press up, you immediately become throw invincible, and then 6 frames later you finally leave the ground and can be thrown (with an air/anti-air throw) again. So those 5 frames you can be hit on the ground.
Source: When I did my testing for all the dash numbers, I also tested all the prejump frames. Only chunkless Aganos (4f) and 3 and 4 chunk Aganos (6 frames) have different prejump.
But yes, everything else SithLord said is true. If you are -4, you can hold up to avoid regular throw punishes because throws hit on the 5th frame (though it won’t save you from Raam’s 4f grab and 1f shadow grab).