Flattened fourth notes are extremely rarely, and is only found in the diminshed whole tone scale (super locrain). Not even the chromatic scale has the spelling flat fourth. It's almost always spelled as a major 3rd. Plus, I was always told it was the flattened fifth that was the devils interval. Just play it, it sort of has a evilish sound to it. And I bet back in those days, if you sat and played it over and over on a church organ. You would probably freak everyone out. :D