There is definitely something Modern about this gem.
With Call of Duty 4, they continue the same multiple characters and multiple nations storyline in a modern setting. You have the SAS dealing with a problem in the former Soviet Union while the USAC dealing with an issue in the Middle East. While the stories start off separate they eventually connect into an extremely thrilling finale. From the first mission until the last one, you are constantly in a shoot-out and every moment is remarkable. Like other Call of Duty games, it's not all on foot, as you'll encounter a thrilling level in an AC-130 Gunship (my favorite) and sections where you are manning a Helicopter Gun.
The game is also filled with a lot of weapons, from small firearms to the massive Javelin; there is something for everyone, but you'll want to mix it up to really experience the game.
The graphics and sound are extremely top notch and if you're not playing this game on an HD TV, then you shouldn't be playing this. The sound works well and really helps create atmosphere. From the bullet sounds, to the talking of the soldiers and civilians you really do feel a part of the action.
The single player campaign is short to some degree. If you play the game on a more difficult setting, you're going to feel it and you will die. In all, the game should take about 5-7 hours, a bit more if you try to find all the extra intel (or if you're an achievement whore).
The multiplayer is also incredible both online and off and there is a lot to do.
In all, this is definitely a must buy for anyone who enjoys a good title. You might complain that the single-player is too short, but when you look at it, the only way they could have made it longer was to add another faction to the mix. But if you do that, then might add unnecessary storyline and then you'd complain about the game being too drawn out. But thanks to the shorter single-player, you'll have more time to enjoy the online, and there is nothing wrong with that.