Adding a new Nas hdd

Assuming you are going for redundancy, not just JBOD, adding a 4TB will give you that -- and nothing more.  A two-drive unit with a RAID5/XRAID confiiguration will have a volume the size of the smallest drive.  But if you have plans to replace the current drive sometime in the future, then you are all set for it to be a 4TB one and double your capacity.

This is why I never quite understood why anyone would buy a 2-bay NAS.  With your corrent configuration, you have to install 2 4TB drives to double your capacity.  In a 4-bay unit, you could installl one more  2TB now for redundancy, another 2TB later to move to 4TB, and one more yet to go to 6TB.