As I have read here there is a chance to add new function in Asus X99-PRO BIOS.
For PCI-EX16_4 slot there is only two options: AUTO and x8 (with M.2 disabled).
I am looking for possibility to enable additional option: x4 (with M.2 enabled).
Thank you in advance for any help.

X99-PRO based on bios v4101 (2019/07/17)

modules and microcodes are updated
it does have x4 option set in already.but as a failsafe
also there are two options visible as follows; auto-x4 and auto-x8
now should be shown as; x8-auto and x4-auto

updating management engine firmware also recommended,
Intel (Converged Security) Management Engine: Drivers, Firmware and Tools

As rule of thumb,keep your OS,apps,bios and drivers updated