What does the useragent "Mozilla/4.0 (compatible;)" mean?
Solution 1:
According to several resources (such as this entry), these are requests used by proxy tools that employ pre-fetching strategies (they go and download all of the pages that are linked from the page a user is visiting in order to cache them for fast retrieval). Blue Coat, Verizon, LookSmart, and others have all been found at one time or another to use that UserAgent in their tools.
Also, according to Microsoft, some versions of Internet Explorer, for historical reasons, have used variations of the Mozilla/4.0 (compatible) UserAgent. See here for more details.