I have a couple of individuals that do want me to add a 4th board support to the mini "officially" -- it's in the list, but is low priority at the moment.
That would be an awesome feature and maybe extend the life of the z9 mini for a bit. I've been eagerly awaiting another release, I hope to see one in the future.
It detects the 4th board and runs it now. The only thing is you wont see it in the web gui until the web gui status.html page in the cgi-bin folder is updated. The web gui on a fullsize already shows the 4th board. How you can tell the 4th is running is look at the hash rate it will be higher then running 3 boards. You can use winSCP and just replace the page now but on a reboot the page will resort back to the origional page.
On the good side using a 4th board reduces the discard rate by about half on the machine
Here is a link to just the status html page....To use it install WinSCP...login to the machine....goto www/pages/cgi-bin and replace the current status html page with the one supplied in the link below....Close it refresh your browser on the miner status and the 4th board will show.
To set the freq in the 4th board it uses the global and the others can be set as usual
https://drive.google.com/file/d/1R-iplyZPd5UZCOo2A3YTQgvYtd7TMYpn/view?usp=sharing