Translation:There is an open discussion about this.
A discussion about a topic: this implies that the discussion was just a conversation, really, and it might not have stayed strictly on-topic.
A discussion of a topic: this brings to mind a true discussion, going into all sorts of details of the topic (and only the topic).
A discussion on a topic: here the discussion might be somewhat one-sided, almost a lecture.
A discussion for is invalid. It's not a thing. Hope I helped^.^