4th level feels about right as an entry point. Enough adventuring opportunity to make a name for yourself and get an invite or come to the attention of an organization that has benefits associated with it.
I like the idea of using the Renown system to establish criteria. I think entry quests/ prove your worth and/or loyalty type situations are also viable requirements for entrance into a prestige organization that has side benefits.
dropping hints and legends about these organizations also gives characters something to work towards and embeds them further in the world, the mechanical payoff is for the players.
