I'll throw in a follow-up "thanks" as well.
After reading the entire thread, I was inspired to build an image with aarch64 and the rpi firmware and replicate a configuration that I currently deploy on an armv7 image on the 3B/3B+.
Starting from a base aarch64 installation from rpi-3-latest, I grabbed the weekly AUR build of the raspberry pi kernel (built from Sakaki's Gentoo-on-RPi stuff), built it, installed it, and rebooted. Simple and easy. Nice!
Then I grabbed moonman's raspberrypi-firmware PKGBUILD and built it, and got access to most of the stuff I couldn't get previously in aarch64. Again, simple and easy. NOICE!
The only thing missing for me was the MMAL/OpenMax bits, which
is actively being worked on.
I didn't realize the level of effort involved in this kind of work prior to reading
this statement and
this exchange between sakaki and 6by9.
So I'll echo graysky's thanks to moonman for the PKGBUILDs, rootfs, etc for the RPi-4, and also throw in a quick thanks to soundcheck for talking through the process in that thread so I could easily use the work for my purposes. This exercise also furthered my understanding about the ARM/RPi architectures that I had been struggling with for a while.
On the shoulders of giants...