partitioning - Install - how to create additional partition (currently: 'unusable space')

How can I create another partition (to e.g. parallel install a different ubuntu version) with this current partitioning table (gparted within Ubuntu install)? After shrinking /dev/sda4 there is only this 'unusable' space?!
My current partitions are as per gparted:

/dev/sda
   /dev/sda1 ntfs                (Windows)
   /dev/sda2 ntfs                (Windows)
   /dev/sda5 ext4                (Ubuntu /home)
   /dev/sda7 ntfs                (joint data partition)
   /dev/sda6 swap
   /dev/sda4 ext4   50028 MB     (Ubuntu 11.10)
   unusable         67000 MB

Any advice?
Could I create /dev/sdb, and if so, how?

Actually using gparted (but not within the Ubuntu installation) shows this as

/dev/sda1
/dev/sda2
/dev/sda3
   /dev/sda5
   /dev/sda7
   /dev/sda6
/dev/sda4
unallocated

i.e. there are already 4 primary partitions.
Therefore: Is there a way of making /dev/sda4 into an extended partition to create the logical /dev/sda8 and /dev/sda9 within? Or could I squeeze a /dev/sda8 after /dev/sda6 if I shrink /dev/sda6?

I reexpanded sda4 to its previous size, shrunk sda5, and was able to create an sda8 as part of sda3.

Answer

see question update and solved part. (ultimately more an issue with the gparted display within the ubuntu installation, which made it difficult to realise the 4 partition layout.)