Hi
You can grab any version of OSMC from 2016 at Index of /osmc/osmc/download/installers/diskimages. We also keep the most recent versions at Download - OSMC. You can definitely go back to Leia.
Hyperion doesn’t work with the Raspberry Pi’s new video stack anymore. See Raspberry Pi 4 with Libreelec v19 / Kodi Matrix, no grabber is working · Issue #983 · hyperion-project/hyperion.ng · GitHub and Pi 4 Kodi 19 hyperion with internal grabber.
Sam