After update to 4.0 bar has changed it's look. Now it fills from top to the bottom, which looks weird and disorientating. Here is screenshot that shows how it looked before, and how it looks now: http://i.imgur.com/UdBxny3.png
Fixed in https://git.xfce.org/panel-plugins/xfce4-genmon-plugin/commit/?id=a41e75b468abb9ae80a1d4207af7e37338e59144.