I began with the instructions here: https://wiki.alpinelinux.org/wiki/Raspberry_Pi I downloaded the aarch64 Raspberry Pi release from alpine's website, extracted the contents of the .tar.gz archive, and wrote them to a blank FAT32 partition on an SD card. I put the SD card into the Rpi 4B, plugged in an HDMI monitor to the HDMI0 port, and a USB keyboard into one of the USB 2 ports.
When I power on the Pi, it shows the rainbow square and the green light flashes seven times, indicating that the kernel image is not found. I checked the config.txt and it points to /boot/vmlinuz-rpi4.
I've followed other guides that recommended certain config settings for the pi 4B, but the best I got was a blank screen and the same seven-flash error indicating that the kernel is not found. I'm at a loss how to troubleshoot this further.