CoD is realistic graphics wise, but other than that, not really. In real life, it takes only one or two shots to drop or kill a person, no matter what the weapon being used. In Halo, yes they have aliens and whatnot, but it's more realistic because the players are wearing a tank on them, so it's obvious it would take more than one bullet to drop them. For everything in Halo (with the exception of a few bugs/glitches) can actually be explained. If you asked a guy who knew a lot about war and science why something works in Halo, he would say, "Oh, well you see it's quite simple *insert long winded and complicated explanation here*" but if you asked a guy who knew a lot about war and science why something works in CoD he would say, "Um... well... uh..."
As far as the OT goes, yes Halo 4 does seem to have borrowed a couple of mechanics from CoD. Maybe one or two. However, CoD has borrowed from Halo (as have many other FPS games) so it all evens out. I honestly don't think H4 is going to play anything like any of the CoD games. I imagine it to be a lot more fun than CoD.