Metal Gear Confusion 4 (MGS4 Spoilers...maybe)

Ok, I'm confused by one event I saw of MGS4 on YouTube. I was looking for sad moments from the Metal Gear Solid series when I came across one video of MGS4. Snake finds himself in a graveyard, talking to Big Boss who is dying. Now, I've never played the old Metal Gear games (the ones BEFORE Metal Gear Solid started), but I thought that Snake had to kill Big Boss in one of those games. So, how is he still alive? Did I mishear about Big Boss being killed in one of the old games, or are they totally ignoring the old games? Can anyone clear this up for me?