I've said this already, but I'll go over it again.
My levels, when they're finished, will be in six sets. Each set is harder than the one before, but that doesn't mean level 1 of a set is harder than level 30 of the previous set; I prefer to overlap them a little. For the preview, I haven't re-ordered the levels in difficulty order; I've left them in set order. Level 15, a very early level of set 5, is of course easier than some of the levels that come before it from set 4; it's meant to be.
As for Level 4, do you mean the old one (Lemming Dilemma) or the new (Close to the Edge)? As I said earlier, I'm abandonding Lemming Dilemma due to unfixable backroutes. If you meant Close to the Edge, well, that one's new so it hasn't had time to find its proper place yet. I might move it around a bit.