Seagate ST4000DM000 Desktop 4TB SATA 3.5" Internal Hard Drive

Every hard drive is like this because of how space is reported. You DO have a 4 terabyte drive. But it reports in tebibytes. So, that 4TB drive will be reported as 3.63TiB. The actual capacity is 4,000,650,883,072 bytes. That is 4TBs, which is reported in a different unit. Terabytes vs tebibytes. Think of it like this: you bought a stretch of land that is sold as 1 kilometer long. But your computer reports it in miles, so it comes out to 0.62 mile long. While it seems like you got a smaller piece of land, it's actually the same size, just being reported in a different unit.