OK, i think i know where the issue is.
If you look at this page:
we have added additional steps when upgrading from 4 to 4.0.1. Basically step 6 was added which was not there before. Please try overwriting your webassist/plugins folder with the webassist/plugins that is located in the install folder of the 4.0.1 zip file. This include the fix to your issue.
I hope this help.