Where did you check the useage? On your phone? Check your self serve account to verify this. As you stated, this shouldn't happen. Also make sure it says 4G or LTE on top of your phone to verify your connected to the mobile network.
Are you referring to Facebook Messenger? Are you having data issues with other applications or just this one? If data is working with other applications, maybe it was just a glitch with the Messenger app and the 4 MB was consumed by something else. Let us know.:)