Not that the OP has laid out the specifics of the "trade" but we're all assuming trade + cash from him. So he's already lost out some resale from the 7D, he's going to be forking out more cash to make up for the difference. Or are you saying that the money he already spent on the 7D won't be considered lost?
And I'm pretty certain you made the statement "they will hold their resale value longer" more as a blanket statement and only now trying to apply it to the OP's situation, where the math still doesn't add up.
Point is, 1D4 is a GREAT camera. It's a better camera than the 7D in almost every way imaginable. If it meets his needs then sure, trade up. But don't think that "resale value" is one of the reasons to do so. The numbers don't lie. Resale value isn't better.