-j Creates the filesystem with an ext3 journal.
Why not an ext4?
Because ext3 is ext2fs with a journal (and the original ext2 does not support a journal), if you want ext4 use mkfs.ext4 or even mkfs.ext4dev. From the ext2 wiki,
... supplanted more recently by ext3, which is almost completely compatible with ext2 and is a journaling file system.