I did have upgrade from 3.x to 4.x, everything I think went well except when I do and apt clean or update I get:
W: GPG error: http://packages.openmediavault.org/public arrakis InRelease: The following signatures were invalid: 0F6BCC17C26BFADC27F221007E7A6C592EF35D13 The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 24863F0C716B980B
W: The repository 'http://packages.openmediavault.org/public arrakis InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: https://openmediavault.github.io/packages arrakis InRelease: The following signatures were invalid: 0F6BCC17C26BFADC27F221007E7A6C592EF35D13 The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 24863F0C716B980B
W: The repository 'https://openmediavault.github.io/packages arrakis InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.