In QuickTime if you go to Windows, Show Movie Information, you should get information about the file under the format line. It’ll probably say something about the audio codec (AAC, MP3, or whatever), then it’ll name the video codec. It’s possible you’re missing it. If it doesn’t have any information for video then it’s possible that’s it’s just an audio file. I’ve seen people on file sharing networks naming their mp3 files avi (probably to dodge the RIAA... who knows?)
You can also get audio/video properties in videolan during playback. it’s under view, stream and media information, under advanced information (it only shows the info while the file is playing though). Like for QuickTime if there’s no video properties, then it’s most likely an audio-only file.
In your case since you said it’s an MPEG-4 file, I’m just gonna guess it’s an M4A file renamed MP4.