That's not quite what I was getting at: that reading you provided (if you interpret it correctly) says that we can't accept the New Testament as, er, gospel, without refusing to accept Deuteronomy 4:2 as gospel. So you've got to pick-and-choose either way.

Not really, the whole story of the Old Testament is to tell about how man sinned, and fell out of the grace of God. And the only one who can come and save us is the Messiah, the Son of God. So the story of the Son of God coming to earth and dying for our sins would be what every man was waiting for, and would be the inspired word of God.