The Reason the Drive cannot be rebuilt is because, as you said, there is a Mirror on Ports 0&2, if both of those drives appear as "Online" Then The Array is Optimal and there is nothing to rebuild, hence there is no option on Drive 4.
I apologize for using the term rebuild earlier. In order to establish a new array using drive 4. Go to it and press spacebar, you will see axx-xx appear next to it. Strike Enter. Then press F10
In the next screen Press Space bar to select the Span then hit F10
Review the options in the Logical Drives Configured Screen and when ready hit Accept (the defaults are generally ok). Then hit ESC and select Yes and hit enter on Save configuration.
Once done, Ctrl-Alt-Del and reboot
Once set, in windows, go to start->run and type diskmgmt.msc. This will open Disk Management. You should see a Striped Area of unallocated space that represents the array you just created. You need to right click on this and format, file system is ntfs. Once this is done you should have a newly Lettered Drive in Windows.