The Tome of Horrors 4 is a decent book for any collection. I'll go over some pros and cons and then overall.
Pros
1. There are a lot of decent monsters here of a variety of challenge ratings and types which are easily usable in any campaign (not too campaign specific). Oozes, plants, constructs, they are all here in fairly equal amounts.
2. They have nice color pictures of every monster. This is a big plus for me as I scan and make counters of the monsters. This one fact has kept me from buying other monster books from other publishers and this one. Color is king for me. The art was all done by the same artist who has a style reminiscent of the planescape series though I'm not sure if it is the same person. It ranges from very good to somewhat stylized for certain tastes.
Cons (and these are somewhat picky)
1. Whoever designed these monsters was very centered on spiders, snakes, scorpions, rats and monkeys. Especially the first two. These creatures are always frightening on a base level but it is too easy to make another spider, snake or variation without it starting to get a little boring. Look for new monster archetypes other than spider and snake.
2. No animal companions rules. We get some regular animals and dinosaurs but nothing is provided for animal companions which is kind of sad since that is always done in all the bestiaries. It could have been a table in the back (if I missed this I apologize).
3. More text. Some of the pages have a lot of space that could have been filled up with descriptive text about the monsters. Even if it was just fluff to explain how the monster feels and hunts it is space that could have been filled. I'm always wanting to read a bit more about the monsters.
Overall: I'm glad I got this book. It has a lot of what I like and while not all the monsters were hits with me, the vast majority were by like 95%. I recommend it to anyone who likes monsters.