Create partition from free space - cannot create more than 4 partitions

I am dual-booting Ubuntu and Win 8.

When I installed Ubuntu, I forgot to format a partition of 90GB.

Now when I tried to create a partition (in gparted), I get this error

It is not possible to create more than 4 primary partitions

If you want more partitions you should first create an extended partition. Such a partition can contain other partitions. Because an extended partition is also a primary partition it might be necessary to remove a primary partition first.

Here's a Screen Shot:

sda1 is that small space that windows loves to create for reasons unclear to me

sda2 and sda4 are windows 8 drives C: and E:

sda3 is my linux drive

the last unallocated 10.2MB? I have no idea where that came from.

But why is Ubuntu counting Window's drives as primary?

But more importantly, how can I get that 90GB back, Win8 Disk Management also gives some error :(

Uwe Plonus's explanation is correct, but gives no solution to your problem. Here's the solution:

1. Boot a Linux emergency disc. (The Ubuntu installer in its "try before installing" mode should work fine; or you can use Parted Magic or something similar.
2. Launch GParted.
3. If any of the logical partitions (those with numbers of 5 and up) are locked (typically with a little key icon, as in your screen shot), unmount them. There should be an option to do this when you right-click the icon. Your swap partition is the only one that's likely to be so affected, and its unmount opiton is called "swapoff."
4. Right-click the extended partition (this is easier in the list in the bottom pane of the window) and select the option to resize the partition.
5. Adjust the size of the extended partition to fill the available space.
6. Create your new partition within the extended partition, which should now contain the unallocated space.
7. Click the "apply" icon (typically a green check-mark).

This is a limitation of the partition table. The partition table has only enough space to hold the informations of 4 partitions therefore you cannot create more than 4 primary partitions.

To have more than 4 partitions you have to create an extended partition. In this extended partition you can create more partitions as needed and can then create more than 4 partitions.

The primary partitions are named sda1, sda2, sda3 and sda4 in your case. The partitions in the extended partition will then be called sda5, sda6 and so on.