The last - by "downvoted threads" I meant "threads descended from a comment at -4 or less" (though there should also be an effect for threads whose most downvoted parent is at -3 or at -2). It's a bit of a pity there isn't a standard name for those.
OK, so to be clear: you're predicting a roughly 50% decrease of the population "comments which are descendants of comments downvoted -4 or more". This at "I'd be pretty surprised if it turned out otherwise", which is my verbal equivalent of 70%. (For 80% it's "I would be shocked" and for 90% it's "I'd seriously question my worldview on the topic in question", for 99% it's "you should really not be messing around with anything remotely connected with that topic, you're dangerous to yourself and others".)
Here...
I just found out that a new website feature was implemented 2 days ago. If a comment is voted to -4 or below, it and all replies and downstream comments from it will be hidden from Recent Comments, and further replies in that subthread will incur 5 karma points penalty. The hiding, but not karma penalty, applies retroactively to comments in that subthread posted before the -4 vote.
This seems to be worth a discussion post since most people are probably still voting things to below -3 without knowing the new consequences of doing so.