RAID 5 - 3 disks, Can I Add a 4th disk?

With a 4-drive NAS, XRAID will never use anything other than RAID1 for two drives (or layers on drives) and RAID5.  If you want something else, you'll have to move to FlexRAID and do so manually.

Yes, if you add another 1TB, it will simply add 1TB to your RAID5 volume.  But if you plan to replace with larger drives soon, use a larger drive now.  The space above 1TB won't be usable until you swap another drive, but you won't be buying the 1TB and "discarding" it later (yes, you probably won't truly discard it since you may have another use for it afterward).  Of course, if the 1TB is one you already have, it doesn't matter.

When installing drives, you really need to assess your current and potential future needs and budget and replace wisely.  Adding the 4th 1TB adds 1TB.  But swapping 2 1TB for 2TB also only adds 1TB and each 2TB after that also only adds 1TB, each time "discarding" a 1TB drive.  If you later need to go bigger than the 6TB you get from 4x2TB, then you'll be "discarding" drives again.  And, up to the point of the largest ones, price per TB tends to get lower when you go bigger.  So go with drives sized for your future to the point budget permits.  2 4TB or even 6TB as the initial swap (adding 3TB or 5TB) could be a better long-term solution, though the initial investment is a bit more.