I can not vote for any single one, as the system I would like would have the first three combined, and selectable by option. Invisible name/lable (if you choose) for characters you have never seen or talked to before. Brackets or color change for unknown people in chat. Color coded lables for characters that you have seen before, but do not know the name of. Another color for those you have talked to, but do not know the name (non-/intro). A color for those you select as known to you (non-intro). And finally, the standard green for those that have introduced themselves.
So, I am going to vote for not-4.