Hideo Kojima just confirmed over Twitter that Metal Gear Solid: Peace Walker will include a 4 Player co-op mode.

@HIDEO_KOJIMA_EN
HIDEO_KOJIMA
4 people can co-op and it is 60 frames. During loading, characters’ dialogues pop up at random.

Plus, he posted a pic of him playing it on 4 monitors on TwitPic.

Metal Gear Solid: Peace Walker will be available in November and will be released for the Xbox 360 and PS3.