I used Gparted to delete the partitions on a HDD as for some reason had used data on it however, after restarting my system it says my unallocated 4TB drive is only 3.64TB.
3.64 TiB is 4 TB.
The drive is 4 TB big, i.e 4*1000^4 bytes. This is the same as 3.63798*1024^4 bytes, or approximately 3.64 TiB.
Disk manufacturers use GB and TB (metric) when writing their sizes. Many software applications, including
gparted, use GiB and TiB (binary) instead, due to tradition.