I don’t know why you can’t farm many disks on Raspberry Pi 4. I am farming with 20 HDDs on my Pi without any issue. And there’s also one time I can farm with 21 HDDs.
And if you also used a USB-C port on the Pi, you could go with 30 drives+ on a single Pi (there’s someone on this forum showing this method but I can’t find the post).
You can check your hubs to see what could be the issue.