You don't know women, sir. There is no woman in the world who would approve of a $4,000 computer purchase. Unless there was a VERY compelling reason (ie Work) for it, but even then its hard to justify. Its just not worth the hassle.
He can always say he got a really good deal on it through a hook up somewhere or got it second hand, etc. There's ways of doing this, as long as the full $4,000 receipt does not surface.
Anyway, OP, have you thought about selling your gift cards for cash? Maybe to friends, co-workers, etc. so you don't lose much or any value?
That way you can bring the cash to the Apple store, and buy what you want. You don't have to convert all $4,000 this way, just enough for your purchase. You can then go to Best Buy and buy the 24" ACD and a few accessories with the remaining gift cards.
Also, I don't know if Best Buy sells Visa gift cards, because if they do you can just get that.
In the future, I would recommend siphoning money off into prepaid Visa cards since they can be used anywhere and you can use a bunch in combination.