Interestingly, book 4 is about 110 pages shorter than book 3, although to be fair book 3 is pretty chunky (260+ pages). I don't feel book 4 is bad value, per se - it's still bigger than a 40k codex, for example, I'm just surprised at the difference.
Book 3 is really 80% fluff, with the rules being for Avatars (which are kinda like epic versions of Masters, a la warmachine. Not necessarily better or more expensive, just different.)
Book 4 is also pretty fluff-heavy, but it also contains rules for 2 important things: mounted models and the Ten Thunders Crew. Mounted models probably won't affect many folks yet, but surprisingly the Ten Thunders will. This is because most all of the Ten Thunders models are dual faction. So, you can use em in a Ten Thunders crew or your existing crew. Note that not all TT members can work for all factions. Eg Kang is Ten Thunders/Arcanist, and so on. Still, it opens up further crew building options, and allows you to buy a couple of Ten Thunders models without buying into a whole new faction.
Finally, there are also rules for effigies, which are wee totemy doohickeys. Can't remember if they're in book 3 or 4. Will confirm when I get home.
I also picked up a retro fate deck. This is surprisingly nice. It's made of very thin plastic, which is good cos it means the cards are waterproof, but bad because they are slippery wee devils. They're styled like regular playing cards, with pips and face cards, but instead of hearts, spades etc they use the Malifaux suits. If you look for it, all the info you need for Mali is in there, too, like 13 damage for a king or light/medium/severe damage etc. but, you can also use em to play poker
