df reports wrong free space for an Ext4 partition

The first thing I would think is that you have deleted files. Using lsof -n | grep deleted will help you. Does the output of that command shows you some files? (maybe you have a huge log file still being written). If you have files opened by a process (a syslogger or something like Apache web server) that are being written into they may be using a lot of disk space and the easies way will be restarting the process owning of those deleted files.

If there aren't any deleted files, could you paste the output of running tune2fs -l ?

Force a filesystem check on next reboot. You may have unrecovered inodes that are holding data.

# touch /forcefsck  # Run as root, then reboot.

source: http://ubuntuforums.org/showthread.php?t=1360204&p=9209650#post9209650