Definitely One of the Best Games Ever, Mgs4 is Prime material.
In this title there are 3 factors that influence considerably in the experience the player will have when he(she) plays:
The first one is the taste for the previous games or at very least experience with them and a fair bit of knowledge about the story, to summarize if one is a fan of the series, in this case the experience'll be enjoyed to the maximum however the small defects are bound to be noticed faster and possibly more painfully by them ( Me for example), since I believe that fans are the ones who demand nothing less than the best with each new installment in a series.
The second one is fondness for cutscenes in a videogame, its a fact that a large part of mgs games consist in cutscenes, 4 is no exception since kojima and his amazing storytelling skills manage to keep fans and non-fans alike glued to the screen with the narrative, which needless to say is far better than "just coherent". Not everyone is too fond of long cutscenes in games but even if you're part of that population, I strongly recommend you to give it a try, those who do like cinematic experiences in games Will find Mgs4 Delightful 8).
The last main factor is affinity for the stealth action style of gameplay which if appreciated will deliver enjoyment regardless the limited knowledge or full comprehension of the plot.
Gameplay: As I mentioned previously, the style is stealth action but with it has some first person shooter elements implemented to dynamize ranged combats, in first view person you can aim to distant enemies accurately, in short ranged combats third person view is in general the best option since CQC (Close quarters combat) it is elementary to leave enemies out of commission without noise nor waste of ammunition, camouflage is an important part of the game since a modern automatic cammo system based on the mimicry of the octopus was included, it automatically changes patterns when the player stops moving to perfectly match with near surroundings, another important aspect is the purchase and customization of weapons trough instant transactions in the menu at the expense of exchangeable Points obtained instantly each an already acquired weapon is picked up ( all the extra weapons are traded for points).
Its also important to take into account the newly added stress bar which measures the stress experienced by the character during tense or dangerous situations, there are many different forms of avoiding stress to increase which harms the performance in combat, for example to stay hidden and try not to engage long combats with the enemy, there are consumable items useful recover health automatically and to alleviate stress like military rations and instant noodles in that particular order.
Graphics: From environments to character models and details in objects, this game despite of not being displayed in native HD resolution (Up-scalable to 1080p) is vastly impressive, the same goes for the physics those dont fall behind either.
Soundtrack : Beautifully Fitting background music for each scenario and lots nice new themes, Outstanding soundtrack which pales only compared to the original for MGS1.
An Ipod is included in the game, you can find new songs from previous mgs games and other music as well scattered through the game.
Story: Deep, Complex, convoluted, compelling. Best one ever told in a videogame in my humble opinion.
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Honorary Mentions
Amazing Boss Fights that resemble those of Metal Gear solid 1, some tough, none easily forgettable.
Fan Service
Satisfying ending.