Bestiary 4 for Pathfinder is now on store shelves and there are some seriously powerful monsters in it. Up to now, the mighty tarrasque (CR 25) was top of the heap as far as monsters go. However, B4 introduces no less than twelve new monsters with higher CRs. Most of these epic horrors come from three groups; demon lords, kaiju and great old ones. Some familiar names return among the ranks of the demon lords, notably Pazuzu (CR 30) and Kostchtchie (CR 26). Sadly, the kaiju do not include names like Godzilla or King Ghidorah, although Mogaru (CR 28) and Agyra (CR 27) serve as approximations of those classic Japanese monsters. Finally, the great old ones derive, of course, from the Mythos and yes, Cthulhu (CR 30) is among them, as is my personal favourite, Hastur (CR 29), the King in Yellow.
Speaking of the Mythos, they have really gone all out this time including Lovecraftian horrors in B4. Besides the great old ones, the bhole (aka dhole), colour out of space, elder thing, flying polyp, mi-go, nightgaunt, spawn of Yog-Sothoth and star-spawn of Cthulhu all make an appearance.
One notable change in B4 is the introduction of mythic rank. Though only a small number of monsters (and not any of the really powerful ones) have one, the mythic rank (MR) is included for use with the new Mythic Adventures rules. I don't own this book so I can't say much more about it other than to note its inclusion. Overall, it's worth a look, especially if you like really high-powered campaigns.