Personally I'd generally recommend the Pro 6 but that may be overkill if you don't anticipate your amount of data you have to grow all that rapidly.
With the Pro 2, 2x2TB would give you a 1.8TB volume.
With the Pro 4, 4x2TB would give you a 5.4TB volume
With the Pro 6, 6x2TB would give you 9TB in single-redundancy or 7.2TB using dual-redunancy (dual-redundancy is available on the 6-bay units and requires at least 4-disks to be installed. It is not available on the 2-bay or 4-bay).
The Pro 4 is a good choice. You can start out with two disks and add more later as you need more space.
Take a look at X-RAID2 in Action
(this shows single-redundancy).