The good news is that by getting the Essentials Collection, you already have the enhanced versions of the games.
Yeah, Substance is the special edition of MGS2 with a whole bunch of extra missions and stuff.
Subsistence is the special edition of MGS3. It's got a new camera system for the main game. Unfortunately, the collection is missing the second disc from Subsistence. It's got the now defunct MGO1, but it also has a bunch of cool extras like more Snake vs. Monkey missions, all of the comedy videos released online, and most importantly, the first really well done versions of the first two 2D Metal Gear games released outside of Japan. They're based on the cell phone ports which were based on the original MSX versions. The NES version of the first game was a joke, and the second game only got a fan translation patch, if that much. You don't absolutely need to play these games to understand MGS4, though. You get all the information you need from these games in MGS1 The in-game dialog recaps the storyline well, and there are even summaries in some menu. It's actually quite thorough.
Twin Snakes is the Gamecube remake of MGS1. This is perhaps the biggest overhaul of any single game the series has got to offer. It's the entire first game with enhanced graphics and gameplay elements taken from MGS2. Some people don't like it because the new gameplay elements don't really mesh with MGS1, the music was changed, the rerecorded voice acting is considered poor by many, and the cutscenes were reimagined with very much a Matrix theme in mind. I love the game, though. You don't need to play it because you've got MGS1 in the collection, but you might want to give it a ride someday.
No, I'm not joking. Sometimes it's mind boggling how things are explained in MGS4. However, it all does fit, and everything else in the game is so awesome the word awesome doesn't adequately describe it. We need a new word in the English language to describe MGS4. I vote for himiashsaksumiyumikulicious.Ouch. I hope you were joking.