There are two ways you can hope to achieve the same basic end, however.
Restore Snapshot: Before upgrading, you should always make a VM snapshot. After upgrading, you should test your environment by publishing a new image and verifying that your software works before deleting that snapshot. If there is a problem, you can restore the snapshot you made, reverting you to the unupgraded state with the previous ELM version. Similarly, if your SAN has a snapshot mechanism, you can revert the ELM VM to a previous SAN snapshot copy. The ELM VM is nothing but a single VM with a large disk, so any mechanism you have for restoring previous backups of that VM will work.
Make sure you never have both an old and a new copy of the same ELM powered on at once. Also, make sure the replacement ELM is using the same IP address; otherwise, your App Layering Agents (on your Hyper-V and PVS servers) might not be able to communicate with the ELM. You might need to reregister your agents with the ELM (see https://docs.citrix.com/en-us/citrix-app-layering/4/install-agent.html).
Export/Import Layers: If you do not have a previous backup or snapshot of the ELM, you cannot take your existing ELM and revert its state. You can. however, deploy a completely new ELM and import the layers from the one you upgraded. Layer export and import became available in version 4.3 (it was a Labs feature in 4.3, so you might need to enable it before you can import). Go to the existing/upgraded ELM and go to Layers, and select Export. Select everything, and export them all to your configured Network File Share. They will be exported into the UnideskExported Layers folder. Then go to the newly deployed, older ELM, Layers tab, Import, and point it to the Network File Share location. Do not point it directly to the Exported Layers folder, we require that directory structure to be present under the folder you select.
This will import all of your layers into the down-rev ELM. This does not transfer your Image Templates or your Connector configurations, so you will need to recreate them, as well as any additional configuration like your Active Directory junction. You will need to manually register any App Layering Agent installations to the new ELM as well. The old and new ELM can absolutely coexist, because they do not both believe they are the same system. They will not conflict.