Error when flashing recovery to Moto G4 Plus Xt1640 - Image not signed or corrupt
Yeah, I had the same error. The US XT1644 variant can't use the boot.img for the XT1643 and below. I was able to root by installing SuperSU. Run this command: 

fastboot boot twrp-3.0.2-0-athene.img

Make sure you copy the SuperSU zip file in the /sdcard/TWRP folder on your device. Then when twrp loads, you can just pick the zip file and install.