If you want to read for the action (read about lots of blood and organs being sliced, people dying, etc), then yes, book 4 becomes boring. However, in my opinion, it does not waver on GRRM's good writing style. In fact, as far as characterization and well written dialogue is concerned, I would say book 4 is an improvement. I really enjoyed the read, just like the first three. It's book 5 that starts getting on my nerves at a few points, although it was still pretty good, and the action does ramp up quite a bit.