.4 File Extension
What is a 4 file?
A 4 file contains a Unix manual page (man page) that likely describes a special, device-related file. For example, mouse.4 is a man page that describes the file used to connect a mouse to the operating system. 4 files can be accessed using the man utility.
Most Unix-based operating systems include a system manual divided into eight numbered sections. The fourth section of this manual typically includes documentation related to special files and device drivers. Most notably, this section contains documentation related to device-related files found in the /dev/ directory. 4 files are named for the file or driver they describe.
In some Unix systems, such as System V, 4 files instead contain documentation of file formats and conventions. These 4 files can still be accessed using the man utility.
How to open a 4 file
In Unix-like OSes, you can open a 4 file by calling the man utility using the name of the file. For example, to open mouse.4, enter man mouse(4).