I can only give you some general guidelines as I do not know what image was running on your ROCK 4, but below assumes you are running a Debian like system (as that is what we provide).
The 2 major things you need are bootloader and kernel.
First make a backup of your SD card.
Then boot the SD card in ROCK 4, install the above 2 packages.
Now you need to run
lsblk to find out your SD card device name. It should be something like
X being a number. It should not have additional partitions like
mmcblkXboot0, as those are unique to eMMC module.
You will first need manually update the bootloader. Run
sudo /usr/lib/u-boot-rock-3a/setup.sh update_bootloader /dev/mmcblkX (replace
X with the number you found above).
You will then need to update the boot entries. Either edit
/boot/extlinux/extlinux.conf file. You should only have one of them available. You need to replace/remove ROCK 4 entries with ROCK 3 kernel.
Power off, and now you should be able to boot into ROCK 3. Some features might be broken as they may need SoC specific configs. So the recommended way is still remake everything from scratch.