To my knowledge there is no additional charge for 4.01 . I have never seen webassist charge for update to an existing release number.
4.01 should be available for you to download in your account downloads under Powerstore 4. The files have been replaced and the new download is 4.01 . There is also a link for instructions on upgrading from 4.0 to 4.01.
I upgraded to 4.01 yesterday with no issue at all.
Hope that helps.