They provide the following rockchip soc-based motherboard support: Pine64,RockPro64, Pine64 ,Pinebook Pro.
Is anyone willing to take the risk to try the .img file provided by netbsd?
Anyone trying to install netbsd on rockpis?
I think you should be able to get netbsd image for rockpro64 and try to run on RockPi4, most likely it should run if it doesn’t then just flash rockpi4 uboot and switch the dtb to rockpi4.
Else just ask at netbsd forum someone might answer you if they plan to have images for rockpi devices or not.
There is freebsd images for Rockpi4 and RockpiE