The x4f part in the .x4f file is just an extension and although there are many common extensions adopted as 'standards' for Windows - like .bmp, .x and .doc - many more are not.
Essentially, it's meant only to be a short description of a file, like .doc files are documents and .txt files are text files.
However, as Benjamin says, any program can open a file with
any name and write data to it and it looks like someone has written a program which just happens to write .x4f files.
As a Google search has failed to find any mention of that file format, it's not a widely used one so I suggest you open it up with Windows Notepad (if you can) and take a look at it. If the author has created the file correctly, there should be a header at the beginning which tells you what the file actually is and/or what program created it.
TDK_Man
