1. What is mp4?
It’s a video file format.

2. What software can be used to view an mp4 file?
Use KMPlayer.

3. How can you see the details (like codecs and bitrate) of an mp4 file?
Use Media Info.

4. How to convert from mp4 to avi?
Use Any Video Converter.

5. How to convert from avi to mp4?
Use Any Video Converter.

6. Where can you read more about mp4?
Here.