A Mysterious Message - Act 4 Story Quest
A Mysterious Message is the first quest of Act 4 and begins by speaking with Tiresias; the first NPC you encounter after going through the portal in Typhon's room. There are a few other NPCs that you can speak to in the City of Rhodes that will update this quest but all you actually need to do is travel to Medea's Grove in the northern Ixian Woods and speak with her to complete it.
Side Quests In Area:
A Crab Story (City of Rhodes)
An Impossible Task (City of Rhodes)
The Torch-Lighter's Gauntlet (Coastal Asomata)
Outpost in the Woods (Ixian Woods)
To reach the Ixian Woods you will want to travel east of Rhodes through the Coastal Asomata area. At the northern end of the Coastal Asomata area you will find a locked gate which says "Overlord's Passage ~ Sealed By Guardian" when you mouse over it. This gate is quite literally right next to the path that leads into the Ixian Woods.
Overlord's Passage, better known as Secret Passage is a very unique area which can only be accessed if you've acquired The Overlord item from defeating Hades on Normal difficulty. This location is one of the more popular farming locations in the game too, for more information check out my The Secret Passage Farming Guide or How to get into The Secret Passage?
When you first arrive at Medea's Grove you'll have to defeat two Ascacophus tree guardians. The gate will open for you once they're dead and you'll be able to speak with Medea and complete this quest. As always, completing this quest will automatically begin the followup, Medea's Price.