Metal Gear Solid 4 secrets

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.

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.

Digital Camera
See Main Article

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.

Scanning Plug
After clocking up 10 hours of play time on Metal Gear Online, the scanning plug can be purchased from Drebin. Used the same way as the syringe, injecting an enemy soldier with the plug hacks into the SOP system and temporarily reveals the location of all enemy guards.

Ammo
Completing the game once will unlock new kinds of ammo and smoke grenades.

Dolls
There are five dolls that can be obtained in the game. Each is obtained by defeating a respective boss non-lethally (for the Beauty and the Beast unit, specifically the Beast form). When all 5 dolls are collected you are rewarded with the Solar Gun.

Act 4 passwords
When Otacon tells Snake to input the code in his old office at Shadow Moses Island, enter one of these codes to get its corresponding reward. If entered correctly, Drebin's monkey will squeak to confirm the code was entered correctly. Note that since there is only one chance to input the code per run-through, only one code can be entered per run-through.

Extras Menu passwords
These passwords are entered at the Extras screen under Password after beating the game once on any difficulty. The items will be made available upon loading your save game or starting a new game (only after Snake meets with the Mk. II).

Emblems
There are 40 emblems available in total, allocated depending on performance (or by fulfilling certain criteria). Some will unlock hidden secrets.

Mk III Gekko Destruction
After getting the power to work in Shadow Moses where you enter a series of numbers for Otacon, approaching the exit that leads to the electric floored hallway will cause a Gekko to appear inevitably. You can use the the Mk. III to activate the power board near the entrance of the elevator to completely destroy the Gekko, assuming that you get there fast enough to leave the Gekko on the electric part of the floor. Approach the power board and when prompted with an icon denoting an image of the Mk III's action, press triangle. The Gekko wil be electrocuted and will be instantly destroyed, and you will get extra Drebin Points.

The Nomad
Onboard the Nomad, there are many easter eggs that can be seen during the Briefing segments.


 * A PlayStation 3 console can be seen in the kitchen and a PSP can also be seen during the Act 2 Briefing. Sunny will pick it up and play Penguin Adventure, the first game Hideo Kojima ever worked on.


 * A model of Metal Gear REX can be seen on Otacon's desktop.


 * While in control of the Mk. II, the tendril can be used to mess around with various objects.
 * Pressing the remote control in the cargo hold will change the viewscreen to a photo slideshow of Akina Minami.
 * Pressing the dial of the radio in the kitchen will play random songs. The songs are different in each briefing segment.
 * You can tease the chicken in their cages. And if you notice their names are Solid, Solidus, and Liquid.


 * If the internal clock of the Playstation 3 is set to 12/25, a Christmas tree can be seen in the center of the cargo hold when the game is paused.


 * During Act 4 briefing, Naomi's necklace hangs on the most left monitor of Sunny's workstation. A picture of Frank Jaeger is in it.

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."
 * Before heading into the Blast Furnace, Snake receives a funny Codec call from Otacon.

Otacon: Hold it Snake. Time to change the disc. I know, I know… It's a pain. But you need to swap Disc 1 for Disc 2. You see the disc labeled “2”?

Snake: Uhh..no.

Otacon: Huh? Oh, wait! We’re on PS3™ system! It’s a Blu-ray Disc. Dual-layered too—no need to swap.

Snake: Damn it, Otacon, get a grip!

Otacon: Yeah, what an age we live in, huh Snake? Wonder what they’ll think of next!


 * After encountering the rats in the vents on Shadow Moses, call Otacon to hear Snake talk about how they helped him in MGS.
 * 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 hurt one of them and call Otacon, he will be disgusted.
 * In Act 3 kill the resistance members one by one and call Otacon each time and see how he goes nuts. Be warned, however, that doing this too many times will result in game over.
 * In Act 4 call Otacon before walking up to the hibernating Gekko. He will tell you that he's never seen this kind of operational mode from a Gekko. Next, walk up to it and let it jump up. Run away then call Otacon again. Snake will be suprised and say he almost had a heart attack. Then they will joke about the creator of the Gekko being a real bastard.
 * If you kill the ravens in one of the rooms that contains Dwarf Gekkos, Otacon will tell you to stop hurting them.

Rosemary

 * Shaking the controller during a call with Rose will cause her breasts to bounce, depending on how you shake the controller. (Grab one of the controller's handles and shake it.)
 * After defeating Crying Wolf, use CQC or shoot at one of the wolf dogs and call Rose. She will then tell Snake that Antisocial behavior and animal cruelty has been linked for a long time, and warns him that he will go through a thorough psychological evaluation if Snake ever tries to harm any animals again.
 * 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 Roy 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 30 Ghosts that can be seen in certain places using the in-game camera. Some are characters from MGS1, whilst some are members of the MGS4 development team. In order to see the ghosts, unlike in MGS1, a photo does not need to be taken in order to see the ghost; they can be seen through the camera's viewfinder, as long as the "Sorrow" doll is equipped.

For a comprehensive lists of all the ghosts, see the Ghosts page.

MGS1 Flashbacks
When Snake goes back to Shadow Moses Island, there are a number of locations or things Snake can do to have flashbacks of conversations or sounds from his mission on the island during Metal Gear Solid. Note: Flashbacks cannot be had while Caution mode is in progress. If a player listens to whole of the flashback (the flashback is cancelled by entering the Codec, the START menu or by moving too far away), Snake will be awarded Drebin points.

Hidden First-Person Views
Some cut-scenes allow Snake to look in first-person view even if the L1 icon does not appear in the corner of the screen, while others have little hidden scenes that happen quickly when the L1 icon is displayed.