It looks like it’s under the following in bios
Advanced->Onboard Devices Configuration
See page 32 of https://dlcdnets.asus.com/pub/ASUS/mb/SocketTRX4/Pro_WS_WRX80E-SAGE_SE_WIFI/E18120_PRO_WS_WRX80E-SAGE_SE_WIFI_BIOS_Manual_EM_WEB.pdf
You’ll want to set it to 4x4x4x4x
Note that most often, the PCIe slot ID closest the CPU starts with 1, and increases from there, however on some boards (Like my Tyan S8030) it’s the exact opposite.
Edit
I just found something else on pg 33,
Use PCIE Raid mode when installing the Hyper M.2 x16 card…
Everyone else calls it Bifurcation which makes for easy searching, but Asus wanted to feel cool instead, so now it’s a pain in the ass to find. This caused problems for someone else here as well.