Jim Butler said:
In my utterly biased opinion, I'd recommend Minions: Fearsome Foes. ;-)
As for creatures 4th level and lower, you'll find more than 50 of them.
You can read more about it here:
http://www.rpgnow.com/product_info.php?products_id=2235
two tries and then an edit.
I'd purchase in the following order:
Book of Templates - Deluxe Edition ($8 pdf at RPGNow) Honestly the best money you'll ever spend for monsters.
Tome of Horrors
Complete Minions ($10 pdf at RPGNow). Includes more, and sells for far less, than the original print version. Being the owner of a print version myself, I'm a little conflicted there...but you can pick up the BoT-DE and Minions for slightly more than half the cost of the FF.
Fiend Folio.
I have a number of other bestiaries, including the Penumbra Fantasy Bestiary, Liber Bestarius, Dangerous Denizens, Ravenloft Denizens of Darkness, Bane Ledger (pdf), Gaming Frontiers: Monsters, and a few other pdf collections I can't place my finger on at the moment. I've also owned the original CC and CC2, but have sold them both. I can't recommend spending the money for the PFB -- I think you'll get more bang for your buck buying two of almost anything else.
Also pick up these free collections.
The Book of Beasts (150 pages;
http://www.geocities.com/Area51/Dungeon/3980/upload/bob/)
The Bestiary of the Realms, V1, by Eric L Boyd and Thomas Costa (142 pages; I couldn't locate a link to this collection, but it was posted on the front page here a few weeks ago)
Cheers!
Nell.