Using ext4 on Gentoo (and probably other distro's)

Keeping it short:
when creating ext4 filesystems with e2fsprogs' (1.41.1 works great for me) mkfs.ext4 I couldn't mount them. The error I found in dmesg was

[ 5994.790520] EXT4-fs: loop0: not marked OK to use with test code.

After a bit of googling I found that to use ext4 filesystems with the current in-kernel ext4dev driver you need to tell the filesystem it can be used by an experimental driver. The option for mkfs.ext4 to do this is -E test_fs. After this you can mount your ext4 filesystems without any issues (not without specifying ext4dev as filesystem with -t though).