I'm in the same boat. I can't complete level 4, despite joining the two purple slimes together.
So what you want to do is jump the two colors when big on top of each other and then you can jump again. It's a sneaky double jump of some sorts.
That's correct! Maybe it was a little more difficult than I had anticipated!
One thing I learned from making puzzle games: Things that come to you naturally mostly don't come to naturally to your players. But I love experimenting, so it was fine for me!