Only 2TB available

No, you have the correct amount.  In XRAID, you have redundancy.  But you only have drive space for redundancy of a 1TB layer.  The remaining 3TB of the 4TB drive has nothing to sync with and remains unused.  So, you have 2x1TB partitions of usable space and 1TB of redundancy.  The basic rule is add the drive sizes and subtract the largest.  If you add another 4TB, then you'll add a full 4TB.  1TB will add to the other 1TB partitions layer and the remaining 3TB will combine with the currently unusable 3TB to form a new 3TB layer.

And, of course, you have to factor in that the NAS uses TiB (multiples of 1024) in reporting size while drives are sold in TB (multiples of 1000), Plus you lose a little to the OS partition and formattng.

If you wanted full use of the drives, at the expense of no redundancy, you needed to go with a JBOD volume, not RAID.