[cedws]

You can extract the tarball to a directory, then run mkfs.ext4 -d DIRECTORY FILENAME BLOCKS to create a filesystem[0]. You'll need to know how many blocks your filesystem needs to be in advance.

Unfortunately, mkfs.ext4 only works on Linux. There is no port for other operating systems.

[0] https://github.com/cedws/concrete-ubuntu/blob/0ae3f076c5a20d...

[guappa]

If you want to create read-only, use ext2. No point in using ext4 if you don't want journaling.

[folmar]

Extents are a useful feature even for readonly although the gain is smaller.

[cedws]

You can also use ISO9660.