How/why have you got 16GB unallocated?
You could just format the unallocated partition as an ext2 or ext3 and then mount it to a directory in mnt. To do this, su to root and format the partition, then make a directory in /mnt like,
mount -t ext2 /dev/hda4 /mnt/partition4
To mount the partition automatically at boot up you need to edit your /etc/fstab file, google can help you with that.
One more thing, your /usr is just one of the directories, you will probably want to back up /home rather than (or as well as) /usr.