There are 5, though admittedly there's a long gap between the first 3 and the last 2 (Thehanu and The Other Wind), and not insignificant stylistic change. There's also the short story collection Tales from Earthsea.
Tales from Earthsea gives a lot of context to the realm and introduces an important character in the plot of The Other Wind. I took it to be the fifth book in the series with TOW #6.