I wanted to migrate from Mender 3.5 to 4.0 on kirkstone and noticed that the artifact verify key was not installed to /etc/mender. I think the reason is that the file is not assigned to the new mender-update package. Instead, bitbake creates a new package mender which gets not installed into the rootfs. I solved this by assigning the file to the mender-update package manually in the mender_%.bbappend:
Note that only the FILES:mender-update part should be implemented in the fix. If you are using the MENDER_ARTIFACT_VERIFY_KEY variable to specify the key file, you won’t want the FILESEXTRAPATHS:prepend or SRC_URI:append lines. And the need to have those lines in a bbappend file are documented in Building for production | Mender documentation