A file with the F4V file extension is a Flash Renamed MP4 file.

In other words, an F4V file is simply an FLV Flash video file that's been renamed to F4V. The F4V format, along with the FLV format, are MPEG-4 video formats defined by Adobe Systems.

How to play F4V files
Many programs open F4V files since it's a popular video/audio container. Adobe's Flash Player, as of Version 9 Update 3 can open F4V files. Many standalone players from 3rd party developers can play F4V files.

How to batch rename F4V files to MP4 files
You can rename F4V files to MP4 from command line. Also there are many batch renaming tools. I usually use Unreal Commander (Windows) to rename F4V to MP4 and other files in batches.

How to Convert an F4V File
Most audio/video converter support MP4 files, so you shouldn't have problems converting F4V files with such programs. You may need to rename F4V to MP4 first.