There's no Rock Pi 4B in the ARM Editions list (21.12)

[nl.smart]

Hi,

You can build the image for your device, not so hard to do.

[Wiki] How to contribute to Manjaro ARM

[Wiki] Contributing to Manjaro-ARM Over the past few years of Manjaro-ARM’s existence, we have had several people offer to help or at least show interest to help. While we appreciate every offer, it seems one of the largest issues that comes about is a large disconnect between the people wanting to help and the procedures to actually get anything done. We have made it extremely simple to do basic functions for manjaro-arm to mitigate the whole “Dont know how to do” or the “Dont know what to do”.…

The devs can’t build the images for every devices.