I need a app for iPhone 4S that I can use to lock my messages?

I need a app for iPhone 4S that I can use to lock my messages?

The only way to restrict access to the phone is by using the native passcode feature. There may be a third party messaging app that allows you to lock messages, but my experience with third party messaging is that they are mostly useless, and require the person you are sending a message to have the same app. And none of the more well known messaging apps have a feature that locks messages. Just password lock your phone and you will be fine. Also I don't know for what purpose you want to lock just messaging, so I'm speaking in generalities.