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 are some of the uncertainties.

• We don't know how large this population is currently. There is a subjective feeling that this number is significant and annoyingly so, but if it is small then it may be hard to detect an effect among the noise.
• We don't know many new comments arise from replies to Recent Comments, as opposed to two people going back and forth, or people explicitly looking for new stuff in a discussion they're following, or people following a particular commenter.
• We don't know how fast low-quality comments get to -4 before they have accrued substantial discussion, or alternately the ratio between number of comments accumulated before getting to -4 and comments accumulated after.

Sadly, I'm about 65% sure that we'll never get to have actual stats on the above, or on the prediction itself. :-/