So I can't find my original source for that one anymore, but I looked at the Chrome Web Store and addons.mozilla.org and they show a total of ≈17m (10mil on Chrome, 6.9mil on Firefox).

I don't see a good active users number on uBlockO's website or anything, and I also don't have a good way of estimating how many of those installs are second or third browsers; but an enterprise install probably wouldn't go through the extension storefronts and would instead be delivered directly via MDM. Whether that means they'd count toward the browsers' totals, I'm not sure.

Still, it seems to me that the vagaries around this probably cancel each other out decently well; sure, some might be double-counted or enterprise installs, but the actual uBlockO users are probably more inclined to be power users, online more often than other users. I'd say that 4% is probably in the ballpark at least. Maybe it's 1%, maybe it's 6%, but I don't think it's terribly far off.