Just to explain where I got the 4 TB figure from: my current understanding is that it is possible to have an almost-2 TB partition start at almost-2 TB into the device, thus it will end just below 4 TB (perhaps two sectors below 4 TiB, although I did not bother doing the math). That's with an MBR partition table. So on a 4 TB device, the first 2 TB may be partitioned arbitrarily (e.g., a few tiny partitions for the OS install, then an almost-2 TB partition for the machine's actual purpose), and the second 2 TB will have to be in one partition. Yes, this is a somewhat weird special case.