There are fake 4TB drives in the market. Maybe you got one? There are some tools available for checking this (I believe, in Debian there is a tool called "f3" or similar.
If there are no important data on the drive, you might reformat the drive to ext2/3/4 and then use "resize2fs" to corect the settings of your drive. In the past I used to grow a partition with gparted, and gparted showed correctly, but df showed still the old size. Resize2fs then did the trick and I got the correct size. If you have Windows on an other computer, there are so,e windows based tools, which are made for testing the real capacity of your drive. Additionally I got such a fake 4TB drive. Although the tools al showed "fake", I could fill it with almost 4TB. Inside was of the case was an USB-stick and I think, the capacity is made by software (shrinking files, double space whatver). However, this stick is (USB-3!) raaaaaaather slow! Just some thoughts, maybe it helps. Best Hans
