Unfortunately there are no Pi4 specific links (yet) in the otherwise excellent guides, only older Pi’s. I think the vast majority with your question will only need to know 2 things.
- Download the image from Index of /osmc/osmc/download/installers/diskimages/v19testing and pick the rbp4 img.gz
- Use the standard Raspberry Pi Imager tool for your OS from the Raspberry website which 99% of Pi owners already have installed and under Select OS, choose the Use Custom option.