These four (4) chapters all have to do with Hezekiah and how Isaiah was the one who told him he was going to die; only to return with news of the added 15yrs. so that his son could be born 3 yrs. later. but I digress..

Scholars, I'm told, have noted how the change at the end of the 39th chapter means a New Author took over with chapter 40, but since the 39th book (Malachi) and the 40th book (Matthew) correlate so well, it doesn't surprise me at all. I usually add about how the 3 and 3-squared and 3-cubed are an added emphasis.