3. ... Bf5 is pretty rare. Out of 27,000 games on my database reaching the position after 3. Nc3 it was played 121 times. The average ELO of the black players was 1,734. The two games with the strongest players were: Rajesh (2402) vs Vaibhav (2589) drawn and Holt (2534) vs Hernandez Carmer (2542) black win. Rajesh continued 4. Nf3 and Holt 4. e4.
Both 4. Nf3 and 4. Qb3 seem to score well, but the games are not between strong players.
The continuation 4. h4 does not appear on my database. So you've got surprise value anyway. I gave the position to Crafty, but only for about 20 seconds and after 3. ... Bf5 it gives the position as +0.75 going down to about +0.2 after 4. h4.
Based on this I think that 4. h4 is fine, it's certainly not instantly losing, it gets your opponent off any opening theory they might have. If you've got some tactical tricks to go with it I'm not seeing huge any reason not to play it.
I'm all in favour of players thinking of and trying out ideas, in fact I encourage it.
But here the risk of weakening the Kingside may prove too much. (see below)
Most 'new' opening ideas in the first few moves have been tried, some have
evolved into recognised variations, others relegated to Capablanca's dustbin.
Look for new ideas in recognised often seen openings from moves 8 - 10.
Something like delaying castling as Black in the Dragon v the Yugoslav Attack,
(very risky) but it saves a tempo for a Black Queenside push and White's
Kingside attack is hitting nothing and they may misplace a White piece or
make a needless (weakening) move on their Queenside.
In this day and age the ideas are more in general plans than one move bombs.
@eladar saidReminds me of a favorite Caro Kann line:
I have started playing this hoping to win a piece when black plays an early Bf5 . I am white. [pgn][Event "Chessboard Editor at Apronus.com"]
[Site "https://www.apronus.com/chess/wbeditor.php"]
[Date "2020.03.28"]
[Round "-"]
[White "?"]
[Black "?"]
[Result "*"]
1. d4 d5 2. c4 c6 3. Nc3 Bf5 4. h4 *
[/pgn]
Is it ok? Or should I continue developing normally?
Edit: a better way for black to play is:
@eladar saidWhich threat is that?
@BigDoggProblem
That e5 push shuts down the dark bishop threat.
@bigdoggproblem saidThe one green pawn demonstrated in his first board.
Which threat is that?