partitioning - Gparted: creating a new logical partition

I have Ubuntu 12.04 LTS and Windows 7 on my computer.
I would create a new logical partition, and when I run sudo fdisk -l I get:

Dispositivo Boot      Start         End          Blocks        Id  System
/dev/sda1   *         2048          206847       102400        7   HPFS/NTFS/exFAT
/dev/sda2             206848        1518526463   759159808     7   HPFS/NTFS/exFAT
/dev/sda3             1910609920    1953521663   21455872      7   HPFS/NTFS/exFAT
/dev/sda4             1585090560    1905090559   160000000     83  Linux

So I have sda1, the Windows 7 boot partition, sda2, the Windows 7 sys partition, sda3, the Windows Recovery Environment and finally sda4, the Ubuntu partition. I have 31.74 gb of not allocated space, but when I try to create a new (logical) partition, it says that I can make only 4 primary partitions. I know, but all my primary partitions are important for me, and I can't swap off no one of these.

How can I create a new logical partition without removing any other partition?

Sorry for my terrible english, and thank you in advance.

P.S.: I have no linux-swap partition.

I believe the short answer is you can't - it's not possible with an "MBR formatted" disk and there is no workaround for its four-partition limit.

Given that, the option of a second hard disk is one resolution.

Be aware that extended partitions, and logical partitions inside an extended partition, are not bootable; an operating system can only be installed on a primary partition and I believe all of the listed partitions have to be bootable ergo, be a primary partition types as opposed to extended/logical.

I've included some reading on MBR {disks} and GPT {disks} from Microsoft and Wikipedia simply because they're good-to-know things and not to suggest using a GPT-formatted disk as an option to your dilemma.

Ref 1: http://msdn.microsoft.com/en-us/library/windows/hardware/gg463525.aspx

Ref 2: http://en.wikipedia.org/wiki/GUID_Partition_Table

Leland