Let me add that there are 8 TB M.2 drives already in 2280 format. So the max capacity of the device (using all M.2 slots) would be 3 x 8 + 4 TB = 28 TB. In addition, you could insert up to 6 1 TB cards in the small extension slots at the sides -> 34 TB.
BTW: it's a fantastic device, very happy with it. Clearly beats Lenovo TPs for example.