Well, lets see.
way large levels.
more complex character models
way more characters and objects on screen.
most likely further view distance.
way better effects. gun fires, explosions, wind, snow...check act 3. actually this act puts MGS4's graphics above other games.
no needing to load menu to switch camo.
Simply put, the scale in 4 is much bigger than any of the previous entries...when talking about technical achievements, don't you also need to put that into consideration?
If MGS4 had a smaller scale like in the old games, or like in Gears or Uncharted, sure, the graphics should be better technically, but with this much stuff going on? This is amazing