I really dislike this. It makes me feel like we all have the responsibility to upvote downvoted threads if we happen to notice discussion going on downstream. After all, if discussion is happening, then it should be greater than -4, and so we should upvote in circumstances where we otherwise would have not voted.
I like the option of not voting. I upvote when I see something I think we should have more of, leave alone the majority of stuff, and downvote only when I see something inappropriate. Our choices are NOT binary, but ternary. Yet this new system of hiding at -4 takes away my choice to not upvote. If I see worthwhile discussion downstream, I feel obligated to upvote.
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.