Open 4FS File
The 4FS extension file is a file system state file used by Puppy Linux's EXT4, or Extended Fourth File System.
Puppy Linux is a lightweight distribution specially designed to run on very old computers and laptops that have an old CPU or very little RAM.
The 4FS file stores the state of the file system at a specific point in time and can be used to restore the file system to that safe state.
This file with the 4FS extension is normally saved on an external partition of the file system, since Puppy Linux can work independently from a CD or ISO, and even from a USB drive.
The first digit of the save state file corresponds to the EXT extended file systems that are used by Puppy Linux (or, for that matter, by most Linux distributions), that is, for example, 2FS, 3FS, and 4FS are used by EXT2, EXT3, and EXT4, respectively.