First let me say, I fell for it; I went out and bought MM4 without really looking at it. It's my own fault and I'll be sure to take my time with anything else coming out. This, for me, is a pretty big downer. There's a few decent critters in here but the majority of it, I probably won't use. Second, I looked and comparedthe numbers from this book to the others, here's a rundown: Pages refer to the actual monster text, no appendix, glossary, or 'how to use this book' pages.
MM1 - 205 monster text pages which includes 394 monsters, this includes the subtypes such as individual giants, dragons, golems, etc.. but each deserves their own entry. Keep in mind this is also all the normal beasts of the wild like bears, whales, big cats, snakes, spiders, etc; as well as the dire versions of many animals and subtypes of lycanthropes (werewolf, bear, boar, rat, etc - which should have their own entry).
MM2 - 200 pages with 136 monsters, including subtypes (giants, dragons, trolls, etc - again deserving of the entries)
MM3 - 197 pages with approximately 142 monsters, not including subtypes
Fiend Folio - 187 pages with 112 monsters
heck even MC: Monsters of Faerun softback has 85 pages with 151 monsters (approx 140 without subtypes)
Rundown for MM4: 192 pages, 110 'monsters' of which only 51 are head entries, meaning the other 59 are subtypes of the head monster.
There are 35 pages devoted to 'Spawn of Tiamat' which includes approx 14 monsters that are dragon-related. (that's almost a 1/5 of the book)
6 pages of Avatars of Elemental Evil?!
45 pages are for subtype/class of previous monsters such as ogres, orcs, gnolls, yugotoths, etc. (Did we really need these? Didn't we learn how to level up and specialize monsters in previous books?!)
11 pages contain 'sample lairs' of which 9 of those take up 1/3 of a page or are full pages.
That's 97 pages right there, people. Over half the monster text of the book!
As for the entries themselves - the new stat block arrangement is changed and is quite messy as all the previous books are done in the same pattern. This one jumps off on its own and looks horrible and not fun to use on the fly. Missing from the stat block is the "Climate/Terrain", you now have to read into the ecology sub-topic to find that out. Included in the monster descriptions are 'sample encounters' and 'typical treasure' for each monsters - this sucks, it's a big waste of space and is nothing but filler. I don't mind the ecology part and the 'knowledge vs' checks could be useful for player knowledge but take up a lot of space too. There's also a filler section for each monster on how these fit into Faerun and Eberron, if you can't do that on your own, leave the game.
Overall, there are a few cool new monsters but in general, this book eats it. Unless you really really have to have that Spawn of Tiamat or can't sub-class your orcs and gnolls, you're just as well to skip this book and save yourself the $35. WotC really jumped the shark on this one, folks.