It's pretty obvious that Solid Snake once again will be the main character and he'll probably be assisted by Otacon, Raiden and Meryl. It isn't hard to recognise him at all. Though he is getting a little old (38-39 years).
It sounds Hideo Kojima allready has planned a lot of work for our hero:
"MGS4" brings a totally new concept and situation, resulting in the toughest "Metal Gear" infiltration mission ever devised. The game is set after the events of "MGS2" and players will reprise the role of top secret agent Solid Snake. The game is slated for the [PlayStation 3], the next generation console from Sony.http://www.konami.jp/gs/kojima_pro/english/004.html
Hal "Otacon" Emmerich
Snake's good friend is also back in business.
I don't know how well her relationship with Snake is going but at least she isn't dead!
Metal Gear Solid 2's main character will also be around for more. With Olga Gurlukovich's Katana in his left hand, and her baby in the other. Looks like he's going after the Patriots!
Thanks to Olga's kid we can conclude that MGS4 will take place only a little while after the happenings in MGS2 (wich took place in 2009).
Seems like MGS4 will be taking place in 2011. Before the story of Metal Gear Acid.
This looks like Naomi Hunter! Just compare the pictures below. Seems like she's back to give Solid Snake a helping hand.
Naomi Hunter in MGS1
The Patriots' henchman is back. And, since he still seems to have both of his arms, so is Liquid Snake! I wonder wich of the two will be in control most of the time in MGS4.
The gay immortal vampire is back for more! He's probably out after Revolver Ocelot to avenge Dead Cell.
I know, I know... This guy should be dead. But then again, that's also what we thought about Grey Fox when we finished him off in Metal Gear 2: Solid Snake. Just compare these pictures:
Big Boss in Zanzibar, 1999
In the MGS1 briefing Campbell said that his body was 'Severly Burnt'... And they reconstructed his DNA from a strand off his hair. But... Then again, there is this pic on the right, that simply must be Big Boss. (He looks too old to be Solidus Snake.)
I have no idea to what his role exactly will be in MGS4.