The ext4 (fourth extended filesystem) is a journaling file system for Linux. It is widely used in many Linux distributions due to its performance, robustness, and advanced features like large file and volume support. macOS: Uses filesystems like HFS+ or APFS. iOS: Uses APFS (Apple File System). Linux: Uses ext4 among other filesystems like ext3, XFS, Btrfs, etc. Windows: Uses NTFS, FAT32, and exFAT, not ext4.