why 4...nd4

i believe you know what my question is.

I would say that is simply a mistake by the opponent.

1 e4 e5 2. Nf3 Nc6 3. d4 is the Scotch, while 1.e4 e5 2. Nf3 d6 isn't - its Philidor's defence. But, you do handle it the right way, because white's most popular move is still 3. d4! The funny thing is that in *both* of these openings, black usually plays 3. d4 exd4, whether either d6 or Nc6 had been played on move 2. I think that its avoiding this exd4, and trying to play both d6 AND Nc6 but alowing d4-d5 that gets black into trouble...

4.dxe5! And blacks in trouble

4.dxe5 actually gives White very little.  4.Bb5 increasing the tension is the path to advantage, as Black will eventually be forced to take on d4 and cede the center.