It was Aljanon's question abou 12 that led to the answer - and just because I sound convinced I'm right doesn't always mean I am! (Though I am wiut lawsuit and A would have said if I was wrong!)
Sorry, Anniec, didn't see your contribution of opus, which, of course, fits with the 4 letters required. I saw p as the second letter and looked no further. Too impetuous I'm afraid.
Colin