Compile EXT4 into your kernel or load as module.

[smburdick]

For example,
   for XFS, enable XFS_FS in your kernel configuration, or compile it as a
   module and load it with 'sudo modprobe xfs'. Most of the distributions will
   have these filesystems already in the kernel/as module.

https://github.com/kdave/xfstests#README

[smburdick]

source of ext4 is already there, checking to make sure it is being compiled at make time

[smburdick]

These were compiled into the kernel

  CC      fs/ext4/balloc.o
  CC      fs/ext4/bitmap.o
  CC      fs/ext4/block_validity.o
  CC      fs/ext4/dir.o
  CC      fs/ext4/ext4_jbd2.o
  CC      fs/ext4/extents.o
  CC      fs/ext4/extents_status.o
  CC      fs/ext4/file.o
  CC      fs/ext4/fsmap.o
  CC      fs/ext4/fsync.o
  CC      fs/ext4/hash.o
  CC      fs/ext4/ialloc.o
  CC      fs/ext4/indirect.o
  CC      fs/ext4/inline.o
  CC      fs/ext4/inode.o
  CC      fs/ext4/ioctl.o
  CC      fs/ext4/mballoc.o
  CC      fs/ext4/migrate.o
  CC      fs/ext4/mmp.o
  CC      fs/ext4/move_extent.o
  CC      fs/ext4/namei.o
  CC      fs/ext4/page-io.o
  CC      fs/ext4/readpage.o
  CC      fs/ext4/resize.o
  CC      fs/ext4/super.o
  CC      fs/ext4/symlink.o
  CC      fs/ext4/sysfs.o
  CC      fs/ext4/xattr.o
  CC      fs/ext4/xattr_hurd.o
  CC      fs/ext4/xattr_trusted.o
  CC      fs/ext4/xattr_user.o
  CC      fs/ext4/fast_commit.o
  CC      fs/ext4/orphan.o
  CC      fs/ext4/acl.o
  CC      fs/ext4/xattr_security.o
  AR      fs/ext4/built-in.a