At CES 2019, Western Digital is demonstrating a prototype of a 4TB thumb drive with a Type-C interface. To be more precise, the form factor is that of a thumb drive, but the Type-C connector currently hangs off a wire. Given that it is a prototype, and has little chance of hitting the market at reasonable price points in the imminent future, this is expected. The main intent is to showcase the technological prowess of SanDisk's flash solutions. As expected, the prototype carries the SanDisk branding.
Based on the form factor, we believe that the product is using the 1.33Tb QLC 96L NAND that started sampling in Q3 2018 [wdc.com].
The article supposes that reads/writes could be in the 20-200 MB/s range. 4 TB at 200 MB/s is 5h33m20s.