By the power of truth, I, while living, have conquered the universe.
People saying Halo 4... -_-
It was Halo 3 that made Halo 4 inevitable by giving absolutely no closure at all and compounding plot points in the final mission telling us that we were about to see the bigger picture.
Note: * refers to plot points introduced in Halo 3
1) 6 of the 7 Halo rings are still primed and ready to fire - a shadow that will forever loom over the Halo universe as long as it's unresolved.
*2) The Gravemind tells us that defeat at Installation 04B only delays his return at the end of Halo 3.
*3) Medicant Bias virtually screams to the player that the Chief's story is not done, informing us that:
*4) The Forerunners are not dead...
*5) ...and that the path to the Forerunners is "frought with peril," suggesting a new threat.
*6) No explanation whatsoever was given to these mysterious new beings referenced by the Didact called the Precursors. Later forming the basis of Greg Bear's 'cosmic game' in the Forerunner Saga, and which we now know is feeding directly into Halo 4.
*7) Nor was any explanation given as to what "following in their footsteps" means when the Didact started talking about the Great Journey he will go on after he fires the Halos.
*7.1) Therefore, The Great Journey does in fact exist, but has been misinterpretted by the Prophets. What is the Journey, and what is the signifigance?
8) Mankind has yet to uphold their destiny as the guardians of the universe (as mankind's destiny is the entire thematic point of the series, not continuing the story would mean voiding the relevance of the franchise as a story). This forms the thematic basis for the Reclaimer Trilogy, as the name itself implies.
9) We do not know if humanity can survive the inevitable tensions between the Covenant client races, the Elites, and themselves.
*10) We did not know (at the time, until Cryptum) what made humanity so special and worth saving in the eyes of Librarian. She said that we held the answer to many Forerunner secrets.
*11) Cortana, who holds the largest wealth of Forerunner knowledge in the universe aside from the Forerunners themselves, is slowly drifting into insanity. How can anyone claim that Halo 3 provided any kind of satisfying closure when we spend the whole game building up to her rescue only for her to be discarded and left to rot into insanity half an hour later?
*12) And finally, Master Chief, the selected messenger of Medicant Bias, is drifting near an unknown Forerunner world at the very end of Halo 3.
Halo 4 covered many of these in detail and has made them pivotal to the central theme of the Reclaimer Trilogy. Halo 4 was certainly needed because Halo 3's faux conclusion was just poor, playing the nostalgia card as so many people point out that "it ends how it started". So what? Almost nothing was resolved at all.