ext4 (fourth extended filesystem) is the open source file system used on many Linux distributions, including Android 2.3 and later. It is a journaled filesystem, like its predecessor ext3. It supports up to 4 billion files, each up to 16 TiB, on volumes as large as 1 EiB.

Though ext4 has improved features over ext3 and some other file systems, it is not a major advance and uses old technology. And while it is still the default on many Linux systems, it is possible that in the future more and more systems will switch to more advanced filesystems, like Btrfs.