I like the way you explore the Slip Zone mechanic in Level 4. It's clever how the box can be used to block the exit.
I found a case when I enter the Slip Zone with low speed, I might get stuck for a really long time. So maybe the Slip Zone speed is better to be set constant.
Very solid start. Just add a context to the Slip Zone --like some kind of magic waterfall maybe because it can move vertically?-- and clever implementation like in Level 4, and I think it will be a very fun game!