1.e4 e5 2.Nf3 Nc6 3.d4 Nf6 4.Bg5? Please let me know the issue with 4.Bg5 in the position after 1.e4 e5 2.Nf3 Nc6 3.d4 Nf6.
I have check the FICS Games Database - Opening Explorer and it seems it's not a popular move. But I cannot understand the reason for this
The issue is that 4. d5 is a much better move; White establishes a strong center with tempo and drives the c6 knight to a bad position.

Besides, 3... Nf6 isn't that popular either (probably because of 4. d5); 3... exd4 is the main line of the Scotch Game.