In the Third Celestial Realm, the members of the Demon Race all seemed quite approachable. Even the more idiosyncratic among them wouldn't go out of their way to start a conflict. Regarding inquiries about a path to the Fourth Celestial Realm, they would tell you everything they knew without reservation. You could say that as long as you asked, they would answer, never deliberately hiding anything, as if it were all just common knowledge.
Thus, Jiang Qiu easily learned some news about the Fourth Celestial Realm from Zi Su. It was almost identical to what he'd heard from the other members of the Demon Race—that no one had entered the Fourth Celestial Realm in a very, very long time, and no one knew how to get there. However, she did mention hearing that the legendary Great Yunmeng Marsh was a place closer to the Fourth Celestial Realm's Heaven-Ascending Dao, and that he could go there to make some inquiries.
