Metal Slug 4 was cool, mainly because it tried being like the very first game. Unfortunetaly, that was not possible due to the changes they made in the game engine since the time. (you cannot massacre 10 soldiers standing in the same place with 10 repeated shots for example, because when one of them dies, his corpse will "hold" the bullets)
The game has ton of nostalgia value and a nice Mission Impossible-esque soundtrack.
Metal Slug 5 on the other hand, was crap. It started along pretty nice, and played pretty nice, up to the first level after which it got stupid and annoying.