Moving disks from Readynas Ultra 2 to RN314

No, you cannot simply transfer the drives. The RN314 uses a different filesystem which is incompatible with the one used by the Ultra 2, so you'll have to copy the data from the Ultra 2 to the RN314. The best way would be to backup the data somewhere else so you can put all four drives into the RN314 and create a new 2x2TB + 2x4TB array (it will have a usable capacity of 8TB, not 6TB), then copy the data from the backup to the RN314.

If you don't already have enough backup storage to hold the contents of the Ultra 2's array, you could put one of your new 4TB drives in a USB enclosure and backup the Ultra 2 array to it, then create a new array in the RN314 with the two 2TB drives plus the remaining 4TB drive (for a usable capacity of 4TB). Copy the data from the 4TB USB drive to the RN314, then pull the 4TB drive out of its USB enclosure and add it to the RN314's array. You'll end up with the same 8TB usable capacity.