Having just upgraded a PC from 3 to 4 I get no last fm logging .. looking in the MM scrobbling log file I see
05/05/2012 14:44:10.186: Last.FM: Exclusion found in "" Filter/Collection
05/05/2012 14:44:10.196: Last.FM: NowPlaying Skipping as Track is excluded by "" Filter/Collection
No idea what filter "" refers to .. however if I actually SET a filter (like exclude video files) suddenly scrobbling starts again. It appears that having NO filters set is a recipe for disaster, since the default was to filter out everything?!