A downvote on a 4-day-old comment is unlikely to be a bot, since it's likely that no one will even see it. If you're seeing agreeable comments being downvoted after several days, it's probably just a grouchy misanthrope or two catching up on HN on their day off.
Is not possible since they turn downvotes off after 24 hours.
/pedant
As someone who incessantly refreshes HN and spends a fair amount of time on the Comments page, I see no reason why it wouldn't be people seeing new comments to older discussions via the Comments section rather than bots. Although I am a demographic outlier in multiple ways, I cannot possibly be the only person routinely cruising the new comments section.
Good point about the new comments feed. I think your parent either misspoke or misunderstood their parent's "[downvoted] comments on 4 day old conversations", which would mean a new (down votable) comment on a thread that started 4 days earlier.
