Update: Forget all that rubbish below.
Turns out in Halo 4 you see the Master Cheif's face, not once but twice! The first is in the introduction cinematic featuring Dr Catherine Halsey where she explains how the Master Chief was abducted from his home and made part of the Spartan Programme. The Chief as a child is featured at this point:
Halo players who beat the Legendary are given this teaser as to what the Chief truly looks like:
It's only a teaser shot of his eyes, but it does reveal a middle aged man.....
Original post below:
The closest one can get is this description which Frankie refers to from the Halo: The Fall of Reach novel...
"If you went down into that line and found a nerd with a deep canon Halo t-shirt and said describe the Master Chief to this police sketch artist, that police sketch artist would then produce a perfectly accurate rendering of an older man, almost painfully pale, almost albino white, with pale blue eyes, reddish hair, close cropped to a skin head, and maybe the last remnants of freckles he had when he was a kid.”
Those with long memories may recall this face from the Halo 3 game however...
|The supposed face of the Chief from the H3 beta build....|
Update: Just like the first Halo ended with a tease of the Master Chief without his helmet, so does Halo 4.