Updated for Pixel 4 XL from my Pixel 3 XL Thread
Like my 3 XL, I unlocked and rooted out of the box but have a grandfathered At&t Unlimited Data Plan. At&t has somehow figured out how to block tethering on a Google purchased and unlocked device. Did a little research, and tethering can be achieved using methods already available for other devices. This can be done entirely on the device. Confirmed to work with At&t grandfathered unlimited, haven't tested others.
Unlocked Bootloader and Root with Magisk(See the How-Tos here: https://forum.xda-developers.com/pix...store-t3989033)
Magisk Module MagiskHide Props Config (available directly through Magisk)
BusyBox Module Installed
Terminal editor of your choice installed on your device
Step one: Make sure you're up to date on the Prerequisites above.
Step two: Open terminal editor and grant SU access by typing
Step six: Open the terminal again and grant SU access
Step seven: type
settings put global tether_dun_required 0
Step eight: reboot
Step nine: test the hotspot and crack a beverage of your choice to celebrate
On my Pixel 3 XL, If I sideloaded an OTA and was just re-enabling this mod, Steps 6-8 should were not required. Quick update(note from my Pixel 3XL thread 4/17/19), just sideloaded the April OTA via ADB. That retained my bypass settings, and tethering works even after re-rooting with no other changes (not even build.prop). it would seem that unless you flash the factory image, you should retain these modifications (but not root, go figure)