OS 4.0 is the highest version currently available for the Organelle 1.
The Github repo you linked to is not a disk image so you can’t flash it to the microSD card.
We are working on getting the disk images on an https site. But for now, you can follow this advice below:
…and download the OS 4.0 for Organelle I: http://thepeacetreaty.org/organelle/diskimages/OG1-v4.0.img.zip
Your browser may stop the download but if that happens, you can ‘allow download anyway’ or similar option from your browser’s download menu/toolbar.
EDIT: see post below.