Hi,
- after 1.e4 e5 2.Nc3 Nc6 3.Bc4 Nf6 4.Bb3, 4...d5 works nevertheless, as well as 4...Bc5. But White is not worse according to computers.
- the 4 Knights with 4.Bc4 (reached by transposition) is better avoided by White, since Black has an easy life after 4...Nxe4 and e.g. 5.Nxe4  d5 6.Bd3 dxe4 7.Bxe4 Bd6 8.0-0 0-0 intending ...f5 -say, 9.Re1 Nb4 and then ...f5) and White has to be accurate to maintain equality. On the plus side, most people think Black is better after 4.Bc4 Nxe4, so you can surprise them if you prepare for this position.

Thanks everybody for their responses.  Maybe I should have pointed out that I only ever reach this possibility by first playing 2.Nc3 and then only after 2...Nc6 3.Bc4, where I then play 4.Qg4 against 3...Bc5, or earlier after 2...Nf6 I don't play 3.Bc4 as I don't want to deal with 3...Nxe4 but White has a number of other third move alternatives, of which I usually play 3.f4.

Though I have considered 3.Nf3, particularly against stronger players, and then going into the Italian Four Knights after 3...Nc6 4.Bc4!?

So back to the lead-up to the line in question, 1.e4 e5 2.Nc3 Nc6 3.Bc4 Nf6 (which I believe to be Black's best response to the Vienna/Bishops, with his knights coming out in this specific order) besides the theoretical 4.d3 which somebody mention, there are probably only 2 decent moves for White, 4.Nf3 transposing into the Italian Four Knights, and the uncharted 4.Bb3, and I guess it's the complete lack of theory that I find appealiing, regardless of it possibly being "uninspiring".

One final thing, the point of 4.Bb3 is to avoid 4...Nxe4, and it seems to render ...Na5 less effective.

And yes, 4...d5!? is an interesting possibility.

Thanks again everybody.  I may alternate between the 4.Bb3 idea, and simply playing the Italian Four Knights, for a while and see which I prefer

Pawnpusher wrote on 06/10/23 at 10:52:23:

I think you must be referring to 3...c6 (a major alternative to 3...Nc6) 4. Nf3 etc.

A couple of books from around 25 years ago (ECO [3rd ed.] and NCO [Nunn]) thought that 4...d5 should lead to an edge for White and that in order to equalize Black should play 4...Be7, but I gather that hasn't been the general view in recent years.

(Speaking of Nunn, just last year he had a game that went 4...d5 5. Bb3 Bd6 6. Nc3 de 7. Ng5 0-0 8. Ncxe4 Nxe4 9. Nxe4 Bf5. Here he played 10. Qf3, varying from the 10. 0-0 he played 40 years earlier and gave as leading to += in NCO.)

@ George,

Apparently even Firouzja fell to the Bishop's Opening - in 15 moves! I couldn't find the actual game but here is GM Smirnov making a video of it on You-Tube. Demidov, M. vs. Firouzja, A. 1-0 Hope you enjoy:

  https://www.youtube.com/watch?v=62UGk7EMkoQ

Black can gambit a pawn with 4...d5

Bb3 (before Na5 makes it necessary) seems more passive rather than prophylactic. Perhaps, 1.e4 e5 2.Bc4 Nf6 3.Nc3 Nc6 4.d3 If 4...Na5 5. Ne2. If 5...Nxc4 6. dxc4 d6. Opens castling and leaves white with superior development, at least so far; White's dark-square bishop has an open diagonal while Black's light-square bishop is useless and Black cannot castle as of yet. Black's open d5 is defended once and attacked 3x. True now c4 is hanging, but only b5 is a threat and not much of one.

1.e4 e5 2.Bc4 Nf6 3.Nc3 Nc6 4.Bb3 Bc5 5.d3 d6 6.f4?? Bg4.
1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.d3 Bc5 5.Nc3 Bc5 6.Bb3 is uninspiring. One Tony Nichols won as Black a game in 19 moves in 2022, though I'm not sure why. IM Gary Lane drew after 6...a6.