Unusable free space?

[Aioanei Rares <debian.dev.list@gmail.com>]

Hi all, I have a HDD (the only one, in fact) with the following layout , 
as reported by df :

/dev/sda2              99G  886M   93G   1% /
/dev/sda1             2.0G  170M  1.8G   9% /boot
/dev/sda5             345G  232G   96G  71% /home
/dev/sda8              29G  172M   27G   1% /tmp
/dev/sda6              59G  5.2G   54G   9% /usr
/dev/sda7              20G  3.1G   17G  16% /var

Now, at the end, I have some 137 GB unpartitioned/unformatted. How can I 
use this free space, since gparted ran from a LiveCD tells me that I 
need to make a logical partition (which is /dev/sda4) in order to create 
partitions anew in that free space. It's probably lack of sleep, but 
please help me out here. :)