The story continues on from the rather misery laden finale of book two, with events also following book three (which ran in line with Bk2). You'll recall that Felisin, younger sister to Ganoes Paran (now master of the deck) and Tavore, adjunct successor to Lorn, has become the focus of the revolt of the 7 cities called the Whirlwind. She names herself Sha' ik, and is partly possessed by the Goddess of the Whirlwind (which is a fragment of a shattered warren). With the ex-priest of Fenner, Heboric, she is part of a rather disparate pack of revolutionaries comprised of warlocks (two of whom are nasty pieces of work), a traitor Fist, and tribesmen. One of her bodyguards, Karsa Olong, becomes a key character and his somewhat grisly story occupies the first chapters of the book and throw some sense on prior events, not least in the Nascent. This fragment of warren also introduces us to the Tiste Edur, Trull Sengar, and latterly the Tiste Liosan. There's more with the undead warriors, the T'lan Imass, and some plot lines on how they've influenced various cultures.
Image from Deviant Art http://slaine69.deviantart.com/art/Trull-and-Onrack-148690529
One such cultural influence is on the Teblor, and we find gradually about this sub-species of thelomen toblakai that are essentially huge barbarians, like Bharghast, or Trell. Erikson's anthropology comes to the fore here, as it does when we get some strange almost time travel scenes later in the book.
Image from Deviant Art http://slaine69.deviantart.com/art/Karsa-the-Toblakai-206542715
It'd be easy with all the background to stifle the book with history and legend. Erikson avoids this in the main part, although there are sections in which the info dump gets too heavy handed. His main virtue is writing down to earth falable characters, and interacting them with gods and epic heroes. I found greatest interest in former Bridgeburners, Kallam and Hedge, and to a lesser extent, Pearl and Cutter. Hedge definitely carries a good part of the book, and given Erikson's habit of doing a George RR Martin I did read his chapters with anxiety. Yet Erikson clearly likes writing his foot soldiers caught up in the epic struggles, and didn't start bumping off characters I like!
As a book it reads well, with enough pace to keep the reader engaged. The resolution had the typically chaotic finale that Erikson likes, as he illustrated in book 3 especially. Plenty of floating plot threads and story arcs are ongoing, although I think book 5 is a sideline into the Tiste Edur. Looking forward to it fleshing out the world a bit more.