Did a "pacman -Syu" on my XU4 before going to sleep -- only new package is linux-odroid-xu3-4.14-141-1
Went ahead and installed it -- installed cleanly. Rebooted. XU4 hangs during boot. My UART module is at work and the XU4 is headless so I couldn't dig too deep into it -- plus its well past my bedtime now!
Ended up attaching the XU4's eMMC to the eMMC-USB adaptor and plugging it into a Pi3. Chroot'd in to the mounted filesystem and downgraded the XU4 kernel back to the previous package -- linux-odroid-xu3-4.13.133-1. Re-installed the eMMC into the XU4 and it boots up normally again.
Will investigate further when I have a way to see the boot console. Will post here if I can track down what might be going on.
Just wanted to post in case anybody else runs into the same issue -- you are not alone.