I remember in one episode in Supernatural, where someone mentions how the Archangels destroyed the Knights of Hell. It got me thinking, because at that point, the only Archangels that we know about that were operating in heaven were Michael and Raphael, since Gabriel was under 'witness protection' and Lucifer was all locked up. Another example, is when Metatron talked about God's departure, and how the 'Archangels' cried for his his return. Again at that point, there were only two in heaven we know of.
I also vaguely recall several other instances of 'Archangels', although I can't cite them exactly. The thing is, would one really use the plural form when referring only to Michael and Raphael? Another thing, is when Metatron called himself an Archangel in one of the tablets, Sam and Dean didn't find that weird (almost like they themselves weren't 100% sure on all of the Archangels).
So my question is, is that were there only 4 Archangels originally, or could there possibly be more we haven't seen?