What you've got is this:

extended partition holding one logical partition

primary partition

primary partition

primary partition where you want to install Linux

This is a rather awkward configuration. The problem is that you've maxed out your primary partition table, so you can't create another primary (or extended) partition; and to create additional logical partitions, you'd need to move and/or resize all three of your primary partitions.

This can be worked around in various ways. Here's what I recommend:

1. Back up all your important data. (You should have such a backup already.)


2. Using a Windows partitioning tool, Shrink /dev/sdb4 so that there's free space at the end of the partition sufficient to install Linux.


3. Download my FixParts program. You can use the Windows version or use a Linux emergency disc that contains it. IIRC, Parted Magic comes with FixParts.


4. Using FixParts, use the r option to convert /dev/sdb5 into a primary partition.


5. Use the l option to convert /dev/sdb4 into a logical partition.


6. Type s to sort the partitions. This will ensure that they're in on-disk order, which will be less confusing than if they're not.


7. Type p to view the partition table and verify that they're all present. (Note that FixParts doesn't display extended partitions, though, so don't be concerned when you don't see one.)


8. If all is well, type w to save your changes.


9. If necessary, reboot into Linux.


10. Launch GParted and use it to expand the extended partition, which should now be at the end of the disk, covering what was originally /dev/sdb4 (and will now be /dev/sdb5), to cover the free space at the end of the disk.


11. Install Ubuntu. It should create new logical partition(s) at the end of the disk.