First, find and talk to the Hierarch. If you've done the Sweet Whispers quest, you'll know how to find him.

He'll tell you to sprinkle Blood of Saints on the four Altar of Saints.

Now, just run around to the four Altar of Saints. The quest journal will give you pins for each. None of them are hidden. NOTE: I don't know if it matters if you talk to the Altars in a different order.

After you talk to the last Altar of Saints, go back to the Hierarch. He'll give you an xp reward, even though the quest dialog mentions "choose your reward."