Just my opinion, of course, but I believe this bestiary is the best of the four bestiary pawn collections. The variety is strong, the artwork phenomenal, and for the most part, Paizo got the scale of the creatures correct this time.
On that note, sometimes, for the sake of including all the artwork, Pathfinder gets a few scales of the creatures wrong. I see that they are getting much better at making most of the creatures around the same scale - although I wish the Wyvaran was a bit bigger (Adlet was the glaring faulty scale on the last box). Shore up a few of these components and Pathfinder is looking at near-perfection. Perhaps including one more size between medium and large would be a good way of dealing with this occasional problem?
The occasional off-scale would be the only fault I have in this box. The creatures are as varied as ever, and I personally believe the artwork is improving on the bestiaries, which is saying a lot considering the artwork on the previous three bestiaries has been outstanding.
If you're looking for a good mixture of characters to add to your rpg campaigns, bestiary 4 is pretty solid and I'd recommend it to gamers who enjoy the artwork and the variety of creatures. I've got say, you did a great job on this one, Paizo. I'll be back for more if the quality remains this high. :)
* box also comes with 20 medium bases; 5 huge bases; and 10 large bases.