Hello @Kamboodle
While you should have 3 HDD slots in the case, you only have 3 Sata ports on the motherboard. Two are taken by your existing hard drives and the third by your DVD drive. So you would have to sacrifice one of those to add the other 4TB drive.
I would instead get an external enclosure for the extra drive and connect it via USB 3.
Example... Amazon basics enclosure