I have found and deleted the cache file in the Users/ path, but to no avail.
I now attach three files which will hopefully assist your research
a. Capture Installer which shows the current extensions installed in DW CS4
b. Capture Shared Folder which shows all the sub directories that exist within the Shared folder. Note there is no Web Assist folder.
c. Capture On Opening CS4 - this is the error message with the path to the missing file which should be in a sub directory of the WebAssist/LayoutManager/Library directory.
As no WebAssist directory exists it is obvious as to why it cannot find the WAconfiguration.js file.
Hope this helps