I can't seem to get up the fourth floor
You have to stretch while standing on a platform (you cannot stretch in the air), and then touch spikes while falling. It's a little bit tricky but you'll get used to it over the course of the game :)
Unless it's not the right floor? This is the one on which playtesters would always spend a little bit more time :)
Also if you are playing on the phone w/ a controller you are going to have a bad time with this unfortunately (though it is still possible to clear the game)