Please take a look at Managing the LXD snap as it contains more information about the “Pinned Feature Channels” approach you are using.
However the pinned feature channel feature only provides “the current and previous releases” for new installs. This is because you will not get any updates when pinned to a previous release and we don’t want users accidentally ending up on an unsupported and non-updated version. This is because the feature is intended for those who want to manage the specific time and order when they perform their updates, rather than pinning at the same version for a longer period of time.
If you’re looking for a LTS stable branch, then the 4.0/stable channel may be of more interest, however you cannot downgrade from 4.9 to 4.0 LTS channel.