I've got MMIV.
I used the Tomb Spiders (& the Swarm & Mummy) to great effect last game.
The book is OK. If you like Monster Books, you can get your money's worth.
It just feels like they tried to do too much. They've tried to reinvent EVERYTHING, a complete transformation of Format, Style and Philosophy, and crammed it all into one book, rather than merely concentrating one 1 aspect to make sure they got it right.
1. I can see how the new stat block could make things easier to run, but it needs to be revised a bit more before that happens. I like how they break it down into 4 sections, with each along a similar theme, it should make things easier. Part of that is me (I've memorized the old format). While I might find it easier to run the monster 'as is' if I want to add a few Hit Dice, modify some numbers on the fly, this new format is a pain in the neck. While it might be easier to run, it's a lot harder to modify quickly. Also, given WotC's past 'success' at correct stat blocks, I have a little bit of problem with the new 'Trust us, it's right' version of HD. (My wife, who plays, but doesn't run, still has problems with how 9HD could equal both 32 and 104 Hit Points). She thinks 9HD is 9HD, thus it should be the same. 9d6 (32) and 9d12+45 (104) she has no problem with.
Making it easier to run a combat, I like that. Making it harder to run a campaign, I don't like that.
2. All the classed monsters. An Orc Barbarian 4 is, in many ways, just another type of CR 4 Melee Combat Monster. An orginal monster could be developed that would basically do everything a a Orc Bbn 4 would. The difference is that it take me about 30 seconds to make an Orc Bbn 4, while creating a whole new monster from scratch could take hours to days (more if I want to playtest it correctly). Also, I have about 30+ adventures, articles, source books that have a sample Orc Barbarian, usually 3-9th level. In short, classed monsters that are 1) easy to create & 2) already can be found/downloaded within 15 seconds aren't really needed.
The classed monsters read, felt, and seemed to me to be a Web Supplement rather than so cool they deserved their own hard-copy pages. (One exception, I really like the Orc War-singer, a Bbn/Bard, if you have to put it classed monsters, give them unique multi-classes/wierd combos that people might not have considered).
Classed monsters should either be 1) Included with the 'Parent Creature' ie, Orc Stats Block then Sample Orc Fighter/Barbarian/Cleric, etc. 2) A web supplement (most of the MM4 would have been better there) or 3) Of such a unique combo or rationale that it really takes the base creature to another level.
3. The much longer entries. I have no problem with this. In Theory. Some of my favorite Monster Books have this format (The Monsternomicon). I like this format. Both the 'Cram as many as we can' and the 'Let's take a couple of pages to fully flesh out each creature' hold equal spots on my Monster Resource Depth Chart. MM4 just seemed to miss the mark here. Using a paragraph to tell me that the CR 6 creature has 2,000 gp of various treasure (just like the DMG Table) is not any good. About half of the treasure entries seemed to be there, just because somebody decided every monster HAD to have a treasure paragraph. Same with the environment. Some was really useful, others just re-stated the old Monster Stat Block Line in 3-5 sentences.
I think if they had just tried to focus on any ONE element for this book instead of trying to re-do everything at once, this could have been a really great, innovative book. Instead it feels cobbled together and incomplete.
I said before, this book COULD be the beginning of trully something great. We might look back on this as the turning point where monster books really became super easy to use & chocked full of all sorts of goodies.
While I can't recommend this book, I can't condem it either. It could have been a really great book (the framework is there). Just the execution suffers in quite a few areas.
All-in-all a mediocre book. It has enough good book, I can't tell you not to get it, but it has enough problems I can't tell you to buy it.