It is possible to use the RockPro64 u-boot to boot NetBSD on the RockPi4c. I suspect I may be the first to have pulled this off because the hardware support isn’t quite there yet.
WORKING HARDWARE
Gigabit ethernet
MicroSD
1x USB 3.0 port
UART
NON-WORKING HARDWARE
(Micro) HDMI & Mini DP
Onboard audio
WiFi & BT
NVME - this allegedly works already under RockPro64 NetBSD
and the other three USB ports.
FreeBSD has support for HDMI 1.X and supports NVME on the RockPi4c so FreeBSD is closer to having reasonably complete hardware support. Hopefully an experienced NetBSD developer will take interest in RockPi4 and make it a better alternative OS option.
Installing NetBSD on the RockPi4
Download and burn:
wget http://nyftp.netbsd.org/pub/NetBSD-daily/HEAD/latest/evbarm-aarch64/binary/gzimg/arm64.img.gz
gunzip arm64.img.gz, use dd or balena etcher to burn the NetBSD image onto a microSD card then:
wget http://cdn.netbsd.org/pub/pkgsrc/packages/NetBSD/aarch64/9.0_2022Q1/All/u-boot-rockpro64-2022.01.tgz
tar xf u-boot-rockpro64-2022.01.tgz
cd share/u-boot/rockpro64/
dd if=rksd_loader.img of=/dev/sdx seek=64 conv=sync
Replace /dev/sdx with the correct device for your NetBSD card. You can now boot NetBSD from the microSD card and login as root via serial console.