Hello viiicky, welcome to Seven Forums!
Here's what to do, first in Windows 7 disk management right click that green "Free Space" and click delete, that should make it all "Unallocated Space".
Then using this method
to open a command window and type the following commands ...
select disk 0
create partition primary
exit - to close the command window
Then use Windows disk management to format the newly created partition.
That should be all there is to create a 4th (fourth) Primary partition for use and be sure to post back with any further questions you may have and to keep us informed.