I would just like to point out that I do like v4 for the extra features, it's just that I miss some aspects of the V3 GUI.
Also, I feel something is missing from both versions : when I preview the files to be synced, I sometimes note that a given file, for instance 'my_very_big_file.mpg', should be excluded from all syncs from now on. All I can do from the preview windows is manually enter a new exclusion rule, I would much prefer to have a goodsync-like feature where I just right click on this item and chose "exclude my_very_big_file.mpg". When the path/file name is complicated, entering it manually is a big cause of error.
Apart from that it really stands out !