The 4-blocks-high platform is indeed a threshold, because 5-blocks is dead height, and the skeleton can jump 2-blocks, which means the player must straightforwardly move to another platform instead of leaping onto it. I understand it can be frustrating when you want to jump to somewhere but then find yourself broken, and it also happens to me when I test the level.
Thanks again for your comment! I will try to improve the tutorial phrase, introduce the mechanics more clearly, and use some decoration to help with the level indication.