Precision 7865 Tower AMD support PCIe bifurcation

With the indication of add-in cards Ultra-Speed Duo and Ultra-Speed Quad, I'd say it does support bifurcation.

https://www.delltechnologies.com/asset/en-us/products/workstations/technical-support/precision-7865-spec-sheet.pdf

It definitely does.  Page 118 of the service manual describes the BIOS setting for it:

What about x4/x4/x4/x4? Is that the same as 1x16, 2x8, and 1x8+2x4?

I'm seeing mixed reviews on this in the bios.

On some systems that support bifurcation, I see it run natively at x4/x4/x4/x4.  While the lane are fixed, the bus numbers allocated to PCIe slots can be adjusted through BIOS option.