Never played 4?
MGS4 was pretty good, more of an emphasis on action and not stealth, but you could still play it stealthily if you choose to. The main difference is that in 4, you can get away with firefights as getting ammo is pretty easy. Whereas in the other games, ammo and supplies are much more limited. The only for MGS4 was amazing though, better than any COD or similar game. There was quite a steep skill curve, but that's also part of what made it so rewarding.