[ Thanks to dave for
this link. ]
“The ext3 or third extended filesystem is a journaled
file system that is commonly used by the Linux kernel. It is the
default file system for many popular Linux distributions.“The ext4 or fourth extended filesystem is a journaling file
system for Linux, developed as the successor to ext3.“It was born as a series of backward compatible extensions to
remove 64-bit storage limits and add other performance improvements
to ext3.However, other Linux kernel developers opposed accepting
extensions to ext3 for stability reasons and proposed to fork the
source code of ext3, rename it as ext4, and do all the development
there, without affecting the current ext3 users”