

Not sure what you mean by unknown biology. I’m assuming the goal is to produce alcohol so measuring density will tell when the sugars have stopped being converted to alcohol because the specific gravity will stop lowering. Assuming proper sanitation has been performed of course to prevent other things growing…
Or do you mean if he’s fermenting not with yeast but like lactofermentation?
+1 for gravity reading.
Also 4 days is nothing, give it a week or two at least.