After seeing the Battle Creek remake, I really hope that Halo 4 will use a Halo 2/3 art style while running on graphics on par with Reach. Here is why that is the best combination:
Halo 3 art style, Halo 3 graphics: http://images.wikia.com/halo/images/e/ee/ThePit_Overlook.jpg/
Halo Reach art style, Halo Reach graphics: http://images.wikia.com/halo/images/b/ba/HaloReach_-_Sword_Base.jpg/
Halo 3 art style, Halo Reach graphics: http://www.gamestm.co.uk/wp-content/uploads/2011/06/HaloCombatEvolvedAnniversary_02.jpg
Halo 3’s art style running on Reach’s engine is beautiful, but the other two options, especially Reach, leave much to be desired.