i used AVS Video Converter with an MP3+G input (a pair of .MP3 and .CDG) to be converted to MP4 format. The output came out
with .M4V format extension. I played it in my PC and played just fine. But why .M4V extension? The hardware I will be playing this in accepts
an .MP4 extension, .MP3, MPEG, etc... no mention about .M4V. I tried renaming the extension to .MP4 but got a warning message that renaming
the extension might not make the file work.

To: groovy3130

It depends on the profile with output settings you selected when converted to MP4. If you converted with output settings for Apple iPod or iPhone, etc, it will give you .m4v extension which apple devices require. You should select presets for Sony PSP or PC standard quality, or others you like. You can also correct file extension by yourself in AVS Video Converter>Advanced>Conversion Options>Output File>FileType.