[PATCH] md: Add '4' to the list of levels for which bitmaps are supported

I really should make this a function of the personality....

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>