4chan doesn't "rank" threads. Like >>91059
mentioned, threads are bumped to the top of the first page when people reply to them without saging. If a person sages in their reply, the thread isn't bumped and remains on whatever page it was originally on, but you would never know unless you actively noted which posts were new to a thread because sages are hidden. You can easily see this when you reply to a thread on a slow board like /vip/. It's less noticeable on fast boards like /v/ or /pol/.