A template or a finished gallery with images?
Templates may or may not be backwards compatible - depending upon the Gallery style. Newer gallery styles (HTML5) are not compatible with older versions.
Finished galleries would not be backwards compatible - you would have to send the images as DNGs back to a Lightroom 4 catalog and build things back from scratch.
In general, it is very difficult to go backwards even one version - let alone two.
Thank you so much.