There's been a bit of a pause in the progression of hard drive sizes due to the limitations of the MBR partitioning scheme which can only handle drives up to 2TB in size. Manufacturers only recently started delivering 3TB drives, and the industry has not yet made a wholesale switch to GPT partitioning and the EFI motherboards that are required to be able to boot from such a drive. So 3TB drives still remain only semi-compatible with most systems - they work fine as data drives if you have the latest OS, but they won't work as boot drives.
I think partly because of this 4TB drives have yet to make an appearance. I'm sure they'll show up within a year, but it would sure help if the motherboard industry could get with the program.