I will say what everyone else said. You have to play all the other 3 games. Though if you play the 3rd one you have already played the best experience you will find the in the MGS series.
The 4th one has some really amazing gameplay and all, but the story and just about everything outside the gameplay was convoluted, unimaginative, or simple. It's also a little short of 10 hours of cut scenes. They all make sense and look fine and there is a story there. It's just that I felt like I was watching one of those bad CGI movies. Spirits Within or Appleseed, though Appleseed was pretty cool. I would not put MGS4 as an example of video games as art since it tries to be a movie, and does not play to the medium's strengths. The 3rd MGS was the best!







