All you non-purists get out of this thread! Rabble rabble!

Wuf, spoilers regarding why you'll hate book 4, without any actual spoilers about the story/characters themselves.
Spoiler:
Book 4 takes half the characters, and talks only about them. Book 5 takes place at the same time, and takes the rest of the characters and talks only about them. Imo, book 5 has most of the good characters. Meanwhile, season 5 will instead take from both books. So your options are really to read books 4 and 5, or read neither and watch the show.