Just finished playing through level 7. I now see why all the concern about level 5. Fail to beat the clock on level 4, go through the long level 5, then level 6 and 7, and you are rewarded with a return trip to level 4 to try it all again. Oh dear. :(
I can really see some less-skilled players NEVER being able to beat the clock on level 4 even if changes are made to that level. Has the following alternative been discussed? Instead of sending the player back to the original level 4 after level 7, send the player instead to a level that is essentially level 4 (more or less a copy if you will) that has sufficient changes made to it (hopefully subtle enough) so that the player should be able to beat the clock without nearly as much trouble as the original level 4, even with the changes made to level 4. Otherwise, players may have the attitude:
"You mean I have to beat the clock on level 4 and if I don't, then I have to do levels 5,6,7, and then return to level 4 again?....the heck with that, I'll just keep trying level 4 till I beat the clock....and if I can't, then the hell with the scenario".
At the time we were discussing beating the clock on level 4, I did not realize that we were going to be sent back to do it again if we failed. Since that is the case, then it becomes even more important that those changes to level 4 give the player a very reasonable chance to beat the clock. Otherwise, you are going to have many frustrated and angry players as above.