An updated "Slackware ARM on a Raspberry Pi 4" installer is now available. A few niggles have been fixed [Nothing to do with Slackware ARM!] with typos and errors in the SARPi README and package description [slack-desc] files. The installer also includes any kernel and/or boot-firmware updates from RPi GitHub. See:
http://sarpi.fatdog.eu/index.php?p=downloads
If you have questions, or need help with any issue(s), please address them here.
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All thanks and gratitude to Mozes, who updated Slackware ARM a few days ago - which made the task of modifying SARPi for the RPi4 unrealistically and relatively simple.
THANK YOU! <3