Michael Ayton wrote on 08/24/23 at 13:02:44:
Seems to me that choice between 4 ...d6 and 4 ...Nf6 is basically a matter of taste/individual choice. I don't see one way as stronger than the other if the aim is to play ...d6/Bd7 + ...g6 lines.
That is my conclusion as well.
After 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O d6 6. d4, Michael states "Black can, should he wish, avoid the 6 ...b5 7 de ending with 6 ...Bd7 (i.e. transpose into 4 ...d6 5 0-0 Bd7 6 d4 Nf6!)."
The ending after 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O d6 6. d4 b5!? 7. de5 de5 8.Qd8 Nd8 9. Bb3 is perfectly good for Black. However, you might not want an ending.
Instead after 6...Bd7 we get to a position covered by Tim Taylor (from 4...d6) in three games (Tal-Ivkov, Stellwagen-Tomashevsky, and Hermlin-Keres) with analysis of additional 7th moves.
Taylor covers several moves for white, but not 7d5. I like 7d5 because White will later be able to play c4 in one move. After 7d5, Black can consider ...Nb8 in addition to ...Ne7.
AOC mentions 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 d6 5. c3 Bd7 6. d4 g6 7. Bg5!?. Kuljasevic continues with 7...f6 8Be3 Nh6 with interesting play.
AOC also mentions 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 d6 5. c3 Bd7 6. d4 Nf6 7. Qe2. Now 7... Be7 8. O-O O-O is an option. It should be noted that such natural moves as 9Rd1 or 9Nbd7 can be answered by 9...Nxd4! with advantage to Black. 7...b5 can also be tried. As AOC notes, Black may not be able to play ...g6. Interesting play seems to be the constant result.