Armored Core 4 comes from a long line of mech action games. The simplest thing to say about Armored Core 4 is that if you liked previous Armored Core games, you will like Armored Core 4. Otherwise, the game is a mixed bag.
AC4 takes place in the distant future, placing you in the role of a lone mercenary fighting huge enemy forces with his "AC" mech - the titular "Armored Cores". These mechs are humanoid in shape and carry weapons in their mechanical arms. If you've ever seen a "Gundam" episode, that's the basis they're working with - think a robot that is basically just a giant person in shape.
The gameplay itself seems promising, but gets somewhat repetitive. The ACs have two weapons (one in each hand) that range from assault rifles to miniguns to rocket launchers to laser guns to laser swords. There are also shoulder-mounted weapons like missile launchers and decoy launchers to lure missiles away from you. Control uses a simple left-stick-moves right-stick-looks system, and it's fairly intuitive. The other main gameplay feature is the booster, which is basically a jetpack that can propel you upwards or rocket you forwards or sideways. The main failing of the controls is that they're not particularly balanced; things that should be connected in purpose are set far apart on the gamepad. For example, pressing L2 uses the boost, but pressing L1 switches your left-hand weapon, and pressing square fires the left-hand weapon. The controls don't seem natural even after you get used to them.
There are a variety of missions in single player mode with some decent gimmicks to set them apart, from "destroy all the enemies" to "get past the enemy's defenses and defeat their anti-air guns" to "drop into hostile territory and maneuver past anti-air guns to reach the targets on the ground". However, the difficult varies way too much from mission to mission - some are just "destroy scattered enemies" with a generous time limit, while others in the same group of missions may have unreasonable time limits or throw you against entire walls of incoming fire. There are some battles against enemy ace pilots, and these are fairly interesting, but the other missions seem to boil down to "shoot all the enemies, who are shooting at you".
The other part of single player is customizing your AC with a wide variety of parts and weapons. Every single part of the frame can be upgraded with a variety of different pieces, from the head down to the legs. Each also carries different statistics, so you can build a thin, fast mech, or a bulky, heavy, high-defense mech. The numbers involved are probably enough to make most casual players shy away - it's difficult to enjoy giant robot action when you have to wade through a sea of statistics to get to it. Besides parts, you can also customize your AC with a detailed set of colors - almost every part can be individually colored, and there are two different methods of picking colors in addition to a pre-set palette. It's also possible to create your own emblems to apply to your mech on various locations.
The graphics in AC4 are decent, but not exceptional enough to be worth it - while the game itself is fairly dated, it's still a PS3 game, and should be a little more impressive than this. Shooting things with missiles and guns feels like it should be more cinematic - as it is, you're basically launching yellow ovals toward the enemy that make weak explosive impacts. The mechs are neat and there's a wide variety, but they're all basically geometric shapes that have been squashed together to make a humanoid shape. The buildings and environments are even more shapeless, with damage effects resembling something from the PS2.
The sound is even more generic than the graphics. The music is hardly worth mentioning - it's just standard futuristic chimes and beeps, without any real independent merit. There's not much voice acting - it's pretty much limited to your Mission Control operator and the voiceovers done for the cutscenes (which is only one guy).
As a whole, AC4 feels like it should be a lot cooler and more fun than it is. Unfortunately, the final product falls short of its ambitions, and feels kind of shallow. Online multiplayer allows for some potential fun, but since the game itself is kind of boring, it can only carry it so far. This game feels most like a 6/10 - good effort, but not enough substance.