I just bought an Optiplex 3090 Micro with a 256 GB SSD installed. I am hoping to replace it with a 2 TB SSD. However I'm confused about which SSDs are compatible with this computer.

I do see on a Dell "Setup and Specifications" document that the following SSD type is listed as compatible with the Optiplex 3090 Micro: 

M.2 2280, Class 40 solid-state drive PCIe NVMe Gen4 x4 Up to 2 TB

This is confusing to me for a couple of reasons. For one, all the other SSDs listed as compatible are Gen3 and limited to 1 TB or less. And elsewhere the specifications say that the computer's PCIe bus supports "up to Gen 3.0". So I'm confused about why only a Gen4 2 TB drive is listed as compatible, when the computer only supports up to Gen3. 

Also, if I enter my computer's Service Tag number and look for compatible SSD upgrades, I see drives that match the Gen3 compatibility listed in the specifications (and only up to 1 TB). But no option at all appears for a 2 TB drive. 

Can anyone tell me which, if any 2 TB drives are compatible with a Dell 3090 Micro, and if so whether it is necessary that the drive be a Gen 4?

Thanks for any help you might be able to give.