I have found that, on the Ars website, they have errata for the 4th edition rulebook, Hedge Magic, and The Wizard's Grimoire Revised Edition.
Does anyone know where errata for other 4th edition products may be found? I'm specifically looking for Heirs to Merlin and The Medieval Bestiary, although I do have other 4th ed products. (I tried Googling, but the only thing I was able to find was the above page).