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. Perhaps it comes down to what signals you want to give to your opponent and what specific lines you're happy to allow/have ruled out.

4 ...Nf6 5 0-0 d6 6 Bc6 bc 7 d4 ed 8 Qd4 Be7 9 Nc3 0-0 is just a transposition to 4 ...d6 5 Bc6 bc 6 d4 ed 7 Qd4 Nf6 (Kuljasevic also mentions the riskier 7 ...c5!? 8 Qd3 g6) 8 Nc3 Be7 9 0-0 0-0. After 4 ...Nf6 White could try 5 Qe2 or 5 0-0 d6 6 Re1, but neither prevents the ...Bd7/...g6 setup. And also after 5 0-0 d6 6 d4, 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!), as played by the heroic Pragg!

Kerangali wrote on 08/22/23 at 11:42:33:


Turning the question around, is there a problem with playing 4...Nf6 first? Sometimes a simple columnar display works best for me. ECO C(4), C79 rows 7 and 8 claim a slight advantage to White. However, after: 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O d6 6. Bc6 bc6 7. d4 ed4 8. Qd4 Be7 9. Nc3 O-O



Stockfish 16 computes the game as approximately equal. I don't think this is bad for Black. Black has to give up on the kind side fiancheto. However, that is also the case after 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 d6 5. Bc6 bc6 6. d4.

FreeRepublic wrote on 07/21/23 at 19:02:08:


Tim Taylor in Slay the Spanish treats the Black fiancheto of the king bishop as something of a universal system as it can come up after either 5.0-0 or 5c3. However, he notes that transposition are not guaranteed.

Certainly 50-0 Bd7 6c3 transposes to 5c3 Bd7 6c3. On the other hand, 5.0-0 Bd7 6c4 and 6d4 are distinct lines. Black, for his part, can play ...g6, ...Bd7 and ...Nf6 by different move orders.

Taylor:  "The basic motif of the Bishop Defence is that Black intends to fianchetto his king’s bishop. Black can play this against almost any move order by White, but must adapt himself to whatever White plays."

Perhaps ...g6 ...Bg7 is not a universal system, but it can be a goal against many moves.

Kerangali wrote on 08/22/23 at 11:42:33:

Thanks for pointing out that course. I was not aware of it. As to your question, one advantage of playing 4...Nf6 instead of 4...d6 is to avoid the line:  1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 d6 5. Bc6 bc6 6d4. This can be answered by 6...exd or 6...f6. With preparation, I think Black can welcome this line.

It seems that Jones' course tries to provide a ...g6 response whenever possible in the double-king pawn opening. It's an interesting idea and I wonder how well he can pull it off!

I prefer PGN or CBV files, but I have delved into the Chessable world. I often play through the moves in Chess Opening Wizard. It's a bit tedious. On the other hand, Chessable expects the user to play through moves anyway.

I realize that I was a little terse in my prior comment, so I will now expand now. My reasoning was that
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 d6 5. c3 f5!? 6. exf5 Bf5
and
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 d6 5. O-O Bg4 6. h3 h5!?
make a good match. They lead to disruptive chess, denying White a simple advantage. 

Other approaches by Black, for example a king side fianchetto, can be attempted against either 5c3 or 5O-O, so why not do so against both moves?

1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 d6 5. O-O Bg4 6. h3 h5!? Here7.d4 is the main move, but it is very complicated. White might try for something simpler with 7. Bc6 bc6 8. d4 (...Qf6) Bf3 9. Qf3 ed4.



Now White can consider 10e5, 10Re1, 10c3 and 10Rd1. Black is a pawn up for the moment but lags in development. His queen side pawns may become vulnerable. He might get something going on the king side with g5-4.

There are several resources available:  analysis at ChessPublishing and elsewhere, games played, and computer evaluations. The computer gives White the advantage for the moment. I analyzed one line with the computer and at some point it recalibrated and gave Black equality. To paraphrase a song by Billy Joel, it never is wrong, it just changes its mind. For now at least, I'll go with human evaluations!

So an ancient main line move gets "!!" ... ah, marketing.

Kerangali wrote on 07/20/23 at 09:06:13:

There's a mention of freezing White's queenside with ...Bd3, which I figured must be a reference to the Siesta (with 7. 0-0).

This screenshot apparently showing Black's ...Qg4-g3 must be an extreme case of threat stronger than execution