According to this, MGS4 will only include one language per region because of disc space.
“Unfortunately because of disc space, we don’t have the space to include other languages, other voice over files for the respective versions,” said producer Ryan Payton. “So the Japanese version’s not gonna have English VO, and the North American and European versions won’t have Japanese VO.”
Obviously, this means if you buy the Japanese version, you won’t be able to understand any of it. Unless you speak Japanese, of course.