I was trying 2x8tb and 2 x 4Tb to = 24TB but it keeps freezing or losing network connection , it formats them fine in a JBOD (Not spanning) config fine. On a side note the EX4 seems to have losing network connection issues anyway, it is out of warranty sadly. tHE MODEL IS WDBWWD0240KBK-NESN
As you are saying that you are using two distinguish capacities drives so you should make two volume.
For more information you can check the product information-
No each drive as its own volume, jbod, not spanning, the model number suggests it can take a total of 24TB, I was trying 2 x 8, 2 x 4.