There is no halo 4, there is a Halo expantion pack called Halo 3:ODST where its takes place right after halo 2 and before halo 3, the reason why ppl get confused by that is that they seem the same, most will be answered in the book Halo: Fall of Reach, Halo 3's ending was Master Chief inside that cryotube, and dont forget the ship in HALO 1 was Pillar Of Autumn, the ship in Halo 3 is , Forward Unto Dawn, and Forward Unto Dawn was cut in half, Halo 3's Legendary ending was him on a PLANET not a halo ring, the elites in halo 1 were enemies and in halo 3 they are allies, the covenant havent discovered earth yet in halo 1 but in halo 3 they have invaded many parts of earth, most noticably africa, Master chief in halo 1 has the MARK V MJONLIR armor, in halo 3 he has the MARK VI MJOLNIR armor,
the the ark was unknown to humans in halo 1 but in halo 3 they attacked and destroyed it.The assult rifle in halo 1 was the Ma5B in halo 3 its the MA5c a more powerful and acurate model of it which was not invented yet in halo 1. i hope all this information has helped. p.s they re-named Halo 3 : recon to ODST