Filesystem Testing Tools
One commonly known simple tool is fsfuzzer and its content is reproduced here.
Its fuller contents are packaged on the MoKB site (customization for ext4 will be needed).
Another is fsstress, which in turn comes from the Linux Test Project.
- See fsstress in action
- prebuilt binary for i386 (2003-04-04, you might to use something more current)