I redownloaded it and Simple 4 still seems to be impossible - I don't notice any changes to it, to be honest. If you hadn't actually fixed it yet (I did notice Cheat Codes are enabled in the new download, so your intention might've just been for people to skip that level for now?), my suggestion would be to get rid of the sign by the exit (on the right) - there's a very clever solution that would work without that sign in the way and still remain quite tricky to discover.
It uses a similar trick to Dodgy 18 of LPIII.
EDIT: Indeed, I checked the timestamps of both the original and "updated" version and they're identical. If you did change the level, you forgot to upload the correct updated version.
For Complex 6, I made this level a long time ago, so I honestly forgot how to 100% it. Sorry(if you want proof, you can check the "last modified" dates, as you may see, I made most of these in early October).
For complex 8, I'll give you a hint:
Don't build over the spinning fire trap. Go under it.
@exit: Do you still update Ultimate Edition to remove backroutes? If yes, I would record my solutions.
@namida: What is the one level that is still impossible?
I just played through most of Ultimate Edition. The lack of hidden traps and the only very few builder-heavy levels were a very pleasant surprise. Even though there are some excellent levels, many of my solutions feel like backroutes. Some comments:
Simple 1: 100% is possible.
Simple 6: The first very good level.
Simple 9: I just made all lemmings from the upper hatch builders until the gap was bridged, which resulted in a rather uninteresting solution.
Simple 10: Faaaaar too many builders: 7 suffice.
Simple 13: Can be solved using only two skills.
Simple 14: Good level, but needs some tweaking to enforce a really interesting solution.
Simple 15: Even I can save 0% ;P.
Complex 1: Rather uninteresting solution for a level showing so much promise.
Complex 2: Can be solved using just 20 builders and no digger.
Complex 6: By far the best level :thumbsup: :thumbsup: :thumbsup:.
Complex 7: Far too many skills.
Complex 11: A trap has its trigger area slightly above the ground, while one would expect it to kill walking lemmings.
Complex 13: Good level, though RR-fiddling necessary.
Complex 14: Another level with a major backroute. 97% possible.