According to ESRB, this game contains: Blood and Gore, Fantasy Violence, Language
Parents should not be terribly concerned. While there are some red pixels every now and then, the gunplay is no worse than the games that were coming out twenty years ago. Think G.I. Joe. The biological aliens explode when shot, leaving no corpse behind. One robotic boss get his neck caught in a closing door, decapitating him, but the description is far worse than the actual experience, and is a direct reference to Contra III from twelve years ago. I honestly do not recall the language being anything worse than what can be heard on network television at 7:00.
Old school gamers will be in heaven with Contra 4. It is the remedy to all progress in the last two decades, for better or for worse.
Gaming historians will be pleased with the museum, the unlockables and the game's ability to give them perspective.
Casual and new gamers should definitely steer clear. This game, quite simply, is not for you.
Deaf and Hard of Hearing gamers will be able to enjoy Contra 4, as there are no significant audio cues throughout.