According to http://www.etymonline.com/index.php?term=ox -- "Oxen is the only true survival in Mod.Eng. of the O.E. weak plural."

From what I can tell, it looks like OE nouns could have either "strong" or "weak" declensions, and this is different from the dual form. Someone on another forum claims that "brethren" is another example.

may well be true, my source is an old State of the Onion talk from Larry Wall, which although a linguist is probably not the most qualified in the world :)

So four ox = oxenen?