Support maybe? As in, we've (Acemagic) tested a bunch of 2TB drives that all work. We have not tested larger.
If you plug in a 4TB drive it should work, but we won't help you if it does not.
Unfortunately you need to either take the risk or find someone else who has.
Only other thing I can think of is not having GPT support and only MBR but that's frankly absurd these days