I found the sudden transition to a pessimistic, depressing tone in Earthsea #4 hard to swallow as well. The criticism of the wizards' arrogance in #4 was indeed overdue but the whiplash was too intense for me.

I strongly recommend #5 (Tales from Earthsea) though, it integrates the new ideas of #4 but more in tune with the original atmosphere of the world. And the plot doesn't depend on having read any of the previous books.