At the end of Halo 3, on Legendary difficulty, Master Chief's Frigate is seen drifting towards an unknown planet. This could very well be Onyx, the "Shield" in the Forerunners 'Sword and Shield". (The sword being the Halo's).
Now in the book Ghosts of Onyx, you discover that several Spartans have become trapped within Onyx, so it is VERY possible that Master Chief will re-unite with former squad mates on Onyx, and battle together once more. The question on WHAT they will be battling is tricky.
There was that fake "leaked" photo for Halo:Reach of a spartan facing a giant worm like creature. This could in fact NOT be a fake for Halo:Reach, and be reality for Halo:4.
Another possibility is that the Spartans will be fighting sentinels (Seeing as the planet is made of them), and possibly the flood. (The Forerunners always seem to have flood contained somewhere, and they always end up being released by some dumbass).
Another possibility is that they could be fighting the remains of the Covenant that were attacking Onyx when the Spartans where there. (However most must have been killed at the end of Ghosts of Onyx).
ANOTHER possibility (Jesus there are a lot), is that they will be fighting indigenous life forms OR other Forerunner created machines. (Different variations of Sentinels).
One last thing, YES there WILL be a Halo 5, and a Halo 6. Halo 4 is marking the beginning of a NEW TRILOGY for Halo.
That's all I have on Halo 4, if you have any speculations or ideas about what it may be, please feel free to post below and discuss.