I would absolutely recommend that you not play MGS 4 until you beat 1-3. Assuming you get an 80GB PS3, you should pick up the MGS Essentials Collection, which includes the first three games for $30.
Or watch the Metal Gear Retrospective on gametrailers. It's substantially cheaper and accomplishes much of the same.
"Be proud of what we make you do." - Master Shake, SGC2C - Baffler Meal Blog time! On Facebook