If number four is in alliance with someone on the Nine Heavens Dark Web, then it doesn't matter who gets the Bone Washing Grass. However, if they're not on the same side, then he would prefer to trade the Bone Washing Grass with number four because they are co-owners of the Rainbow Heart Lotus Fragments and should assist each other.
However, the prerequisite to trade with number four is to make sure they have the North Ming Black Stone. Considering Qingxu Zi has gone missing, he can't communicate with number four directly via the fragments.
The next day, the person who had private messaged Su Chengyu about trading couldn't hold back and took the initiative to contact Su Chengyu again.
"Senior, if you feel that a single North Ming Black Stone is not sufficient to exchange for the Bone Washing Grass, you're welcome to propose any other terms. If they're reasonable, I'm willing to do this trade with you."
