MGS4 Secrets/Unlockables

Bandana

The Bandana can be obtained by finishing the game without killing anyone or by buying it from Drebin for 5,000,000 Drebin Points (4,000,000 DP on Wednesdays or Sundays) or 2,500,000 (No discount on Wednesdays or Sundays though) in Act 5. When worn, Snake will have infinite ammo.

Digital Camera

The Digital Camera can be found behind Sunny's desk during Act One, Two or Three's mission briefing, or in the kitchen during Act Four's Mission Briefing. The camera allows you to take pictures in-game and access them from the photo album in the main menu which can then be exported to the PS3s HDD. If you pick it up during Act 4, you will find pictures of Sunny and Naomi on the camera.

The White Room

After defeating the Beast form of any of the bosses, they will emerge in their beauty form and pursue you. If you avoid them without defeating them for roughly 2 or 3 minutes, you will be taken to an infinite White Room where they will continue to pursue you, with a 3 minute countdown being shown. You are unable to call Otacon or Rosemary while in this room. Equip the camera and the beauties will pose for you. Equipping the iPod and playing certain songs will make them dance along.

Stealth Camouflage

The Stealth Camouflage can be obtained by finishing the game without entering an alert phase or by buying it from Drebin for 5,000,000 Drebin Points (4,000,000 DP on Wednesdays or Sundays) or 2,500,000 during Act 5. When worn, Snake's camo index will always be 100%. However, Gekkos will still be able to see Snake and PMC soldiers will still hear his footsteps. Also, during an alert phase, the camo index will be 95%, allowing PMC soldiers to see Snake.

Weapons

Long Barreled Desert Eagle = Earn the Fox emblem.

The Patriot = Earn the Big Boss emblem.

Race (Ricochet) Gun = Beat the game once.

Railgun = Defeat Crying Wolf.

Type 17 (EVA's gun) = Earn the Hound emblem.

Solar Gun = Collect the five dolls hidden throughout the game.

Thor 45-70 = Earn the Fox Hound emblem.

Codec:

Otacon

When in the canyon before entering the Warhead Storage Building on Shadow Moses, you can call Otacon to hear a somewhat funny conversation. It starts out with Otacon asking Snake how he defeated Raven and his tank during the first Metal Gear Solid. After Snake responds with "Grenades," Otacon says he asked an officer in the Army for advice on an infantrymen taking on a tank. The Army officer responded with "Don't." It is then that Otacon professes what a badass Snake is, going so far as to call him "The Shit."

During the battle with Screaming Mantis, change the controller settings to another port to hear a Codec conversation between Snake and Otacon about Snake's inability to move.
Otacon has a story about virtually every room in Act 4 that Snake encountered in the original MGS. Sometimes you have to call him twice to hear it.
After you beat Crying Beauty, a pack of wolves will appear. If you kill one of them and call Otacon, he will be disgusted.
In Act 3 finish the resistance members one by one and call Otacon each time and see how he goes nuts.

Rosemary

Shaking the controller during a call with Rose will cause her breasts to shake.

After defeating Crying Wolf, kill one of the wolf dogs and call Rose.

During the battle with Screaming Mantis, Campbell (through Rosemary's codec) calls Snake to tell him to change controllers like in MGS1. However, Rosemary then tells Campbell that it will not work.

While fighting Screaming Mantis, call Rosemary to hear Colonel Campbell's suggestion to shoot the Psycho Mantis bust.

If you call Rosemary several times, you will hear a funny story between Rose and Colonel Campbell about how terrible Rosemary's food is.

Call Rose after you die and continue. Snake will tell Rose about how he keeps having dreams about dying. You can do this multiple times, and get different responses.

Call Rosemary too many times and she will ask Snake "What has gotten into you"?
Each day you play the game, Rosemary wears different sweaters.

Ghosts:

As with some Metal Gear Solid games, there are several Ghosts that can be seen by taking photographs of certain places using the in-game camera:

In Otacon's old lab, take a picture of the Policenauts poster. The ghost here is Hideo Kojima.
After beating Crying Wolf, take a picture of the doorway to the right of the doorway which leads to the Blast Furnace or the wolf who appears after you kill/KO her beauty form. The ghost here is Sniper Wolf.

Take a picture of Liquid's crashed Hind D.

Take a photo of the hallway where Gray Fox killed all a bunch of soldiers just before Otacon's office.

Take a photo of REX's right foot to see a ghost of Gray Fox and take a picture of REX's cockpit to see Liquid Snake.

In the bridge after you fought Vulcan Raven in MGS, take a photo to the waterfall
In Otacon's old lab, take a picture of the supercomputers. The ghost here is the Arms Tech president, Kenneth Baker.

In Otacon's old lab, take a picture of the cubicle to the right of the entrance.
Take a picture while standing on the Heliport.

In front of the the elevator in the Tank Hangar is a ghost of Decoy Octopus.

In the hallway which contained the laser/gas trap from MGS1, take a photo of the bottom right corner whilst in it.The ghost here is the DARPA chief Donald Anderson.

In the lower vent which leads to the tank hanger take a photo of the steel directly in front of Snake before the very first right.The ghost here is of Master Miller.

In the canyon where Snake fought the tank, take a photo of the rock area to the left of the door which leads to the nuclear warhead storage building. The ghost here is Vulcan Raven.

In the Hallway with the Electrical floor, take a picture of the hole the GEKKO jumped through on the ceiling.

Complete List at: http://metalgear.wikia.com/wiki/Metal_Gear_Solid_4_Secrets