The most photo-realistic graphics out there, and amazing, addicting game-play!
GRAPHICS:
Like I have said twice by now, you won't find more photo-realistic graphics in any game so far, in my opinion. I already talked about the Chernobyl levels in campaign (there is also one map in multi-player that is based off of it) being such. but if you look at the graphics of the weapons, helicopters, tanks, Harrier-Jets, and more, you'll find little difference between that and their real-life counterparts.
There are little or no flaws in lighting and shadowing in this department, either. Those, too, are immensely realistic and only add to the realistic graphics that Call of Duty 4 almost invariably offers. The guys at Infinity Ward obviously spent a ton of time detailing their graphics to the max. The vehicles, weapons, people, and buildings in this game show it.
GAME-PLAY:
Like pretty much any other game, there are some flaws, but Call of Duty 4 tries its best to exterminate such.
I am going to start with single-player campaign. Campaign offers a very good story. You get to play as the SAS, as Sgt. John "Soap" MacTavish, straight out of selection into the 22nd Regiment of the SAS. The SAS is a special-ops tactical force for Britain that relies on stealth to complete objectives. You also get to play as the US Marines for a little bit of the game, too, where they mostly rely on straight-forward large-scale assaults to complete their missions in this game. As the Marines, you play as Sgt. Paul Jackson of the First Batallion. I don't think he is new, as judging from the dialogue I have heard of him, he has probably been with them ever since he enlisted. I will try not to give any spoilers, so I will just say this: Khaled Al-Asad has killed President Al-Fulani of an unnamed Middle Eastern country. The SAS and Marines attempt to stop him, until things take a huge turn about midway through. That is all I will say about campaign. Like I said, the campaign will not disappoint you. It is relatively short, but it is worth every second of it. Not to mention that trying to beat it all on Veteran difficulty may lead to some less hair by the end of the day, if you know what I mean. Unfortunately, though, there is no co-op of any kind.
In Online Multi-player, you will, of course, find some aspects unbalanced. There are Rifle-Mounted Grenade Launchers that you may find overpowered when overused. But then again, there isn't much they can do about that without taking away the realism of Grenade Launchers. Like pretty much any other First Person Shooter online multi-player, there will be guys who you consider to be "noobs" here and there taking advantage of things, being cowards, and whatnot, but there isn't much Infinity Ward can do about that, either. Another negative is that you cannot have LAN parties, but that is relatively minor, depending on your preference.
Game-play in Call of Duty 4 is VERY realistic in most aspects. Online, you may complain about a couple of unrealistic aspects, though, and that takes a slight hit to the game. There is a perk called "Martyrdom" that lets you SOMEHOW drop a cooked grenade as you die, no matter the circumstances of the death, as if you were Allah and saw it coming, so decided to pull the pin 2.5 seconds beforehand. There are a couple of other such minor aspects, but other than that, game-play runs very smooth, and weapons just feel right. Infinity Ward did a very good job at balancing out online multi-player to satisfy you in many ways. There are a couple flaws that I have mentioned, though, but are relatively minor and petty.
SOUND:
I know that this is starting to sound like an overused word, but I must say again: "realistic". Sound is very realistic. Helicopters sound like how they should, dialogue is great and only adds to gameplay, and the music adds to the feel very nicely during campaign. Guns also sound like how they should sound in real life. Must I go on?
CONCLUSION:
If you couldn't infer it from what I have been saying, GET THIS GAME ALREADY, if you have been considering it. There is little reason to rent this at all first, as it will only shed a little extra money and waste a little of your time. You will not be one bit disappointed with this game at all.