Halo 4 is the first game in a new trilogy with new writers, and a new studio. Halos 1-3+ Reach/ ODST follow a storyline that's pretty well contained in itself. Those games left enough open ends to the storyline of a few characters (namely Master Chief) to allow for future titles. Halo 4 marks the first game in what's known as the Forerunner saga, and it's not 100% necessary to know the story of original games. So, it helps to play the older games, sure, but you won't be too lost without doing that.