I got it working, but be warned! https://github.com/zynthian/zynthian-issue-tracking/issues/218
I followed these steps
https://www.raspberrypi.org/forums/viewtopic.php?f=56&t=279470&p=1692544&hilit=pi4+boot#p1692544
but where the poster goes “Also note that you must have already updated the eeprom on the raspberry pi 4” the relevent part is https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2711_bootloader_config.md
It’s not enough to update the firmware, you need to update the boot config as well.
I did it with this line
sudo rpi-eeprom-update -d -f /lib/firmware/raspberrypi/bootloader/stable/pieeprom-2020-06-15.bin
the output of vcgencmd bootloader_config needs to show BOOT_ORDER=0xf41
Also https://github.com/zynthian/zynthian-issue-tracking/issues/219
