Quota Bar for ownCloud was one of the most popular apps on apps.owncloud.com. Unfortunately Simon Kainz (the initial author) hasn't updated the app for ownCloud 4.
Some days ago I pushed a revised version with new features to Github.
After enabling the app in the Admin Panel, go to your "Personal Setting" and activate it:
The Quota Bar should now appear below the app menu:
Feature requests are welcome!
PS: Simon, please contact me to release it at apps.owncloud.com