I was replaying an old Halo 3 cutscene, and after Primordium, I have just realized something under my nose that I missed.
THIS THREAD HAS SPOILERS, READ AT YOUR OWN RISK.
As of Primordium, 343GS is alive and has taken over a ONI ship, possibly the UNSC Infinity, and is heading towards the Librarian's location, who may be alive.
He states that "I know where to find her".
Ok, Primordium also revealed that Installation 00 isn't the only Ark, the Greater Ark also exists, which is bigger than Installation 00.
http://www.halopedian.com/Greater_Ark
All relevant information on it may be found here.
Since the Librarian is greatly interested in life, and the fact that the Greater Ark is located outside the galaxy, it would be the safest and most logical place to house the remaining Forerunners and/or the Librarian.
Now, you are wondering, how is this relevant to Halo 4? How is this relevant to Halo 3?
Well, according to Primordium, the Greater Ark is lit by greater pillars of plasma, which look very similar to the pillars we see in the Halo 4 concept trailer, although those look dormant.
Still confused about the relevance to Halo 4?
The Legendary Planet may in fact be the Ark coiled up, with the hole representing the top. We know the Greater Ark has 6 arms, and the concept art for Halo 4 and Primordium both show a curving object, which cannot be a Halo ring. It is the Greater Ark, as mentioned in the book.
Now, to link this all together. 343GS, Master Chief and Cortana, Greater Ark, (clearly described as different in both Cryptum and Primordium compared to Installation 00), and the UNSC Infinity.
To end this topic, I would like to say that it is amazing how little pieces of information shown and stated in Halo games have been rubbed off as irrelevant, only to appear again and be of importance, like the Index Cortana took in Halo CE, it saved Arby n the Chief. (Pun Intended)
This is one example, and now, I present you another, something which seems to have a deep complication for Halo 4, 343GS may have known about it...
"I always thought it was part of a shield installation, but it seems I may be mistaken....."
Spark always thought it was part of a shield installation! (Since the monitor's information is limited, he may not have known about the Greater Ark, even if he did, the Terminals reveal he let go links to his former station, explaining his lack of knowledge) Guess what that means... (Let me remind you that the Didact built the shield worlds, it would explain his presence in the Reclaimer Trilogy)