In his book
The Slav (Gambit, 2001), Burgess writes:
Quote:4...Bf5?! was played in some high level games in the early part of the 20th century, but has for a long time been considered very dubious, and is no longer played at the master level. However, it is surprisingly popular at the club level (do some Slav players really start making it up as they go along as early as move 4 in the main line?) , so it is worth investigating in some detail. The main problem is that Black leaves b7 too exposed and in some lines there are problems with the a4-e8 diagonal.
(I love his parenthetical remark.)
4...Bf5 is definitely an inferior move, though probably not quite a blunder (with an accurate followup it doesn't directly lead to a losing position). After 5.cxd5, Black should play 5...Nxd5 rather than 5...cxd5 but White still gets a very nice edge.
Another link where this line is mentioned:
http://www.rci.rutgers.edu/%7Egoeller/kenilworth-pgn/summer05/massey-lecture.pgnAnd here is the famous game mentioned by Smyslov_Fan where White played 5.Qb3 instead of 5.cxd5:
Janowski,Dawid Markelowicz - Capablanca,Jose Raul [D15]New York Rice final New York (3), 1916 1.d4 Nf6 2.Nf3 d5 3.c4 c6 4.Nc3 Bf5 5.Qb3 Qb6 6.Qxb6? (better in 6.c5) 6...axb6 7.cxd5 Nxd5 8.Nxd5 cxd5 9.e3 Nc6 10.Bd2 Bd7 11.Be2 e6 12.0–0 Bd6 13.Rfc1 Ke7 14.Bc3 Rhc8 15.a3 Na5 16.Nd2 f5 17.g3 b5 18.f3 Nc4 19.Bxc4 bxc4 20.e4 Kf7 21.e5 Be7 22.f4 b5 23.Kf2 Ra4 24.Ke3 Rca8 25.Rab1 h6 26.Nf3 g5 27.Ne1 Rg8 28.Kf3 gxf4 29.gxf4 Raa8 30.Ng2 Rg4 31.Rg1 Rag8 32.Be1 b4 33.axb4 Ba4 34.Ra1 Bc2 35.Bg3 Be4+ 36.Kf2 h5 37.Ra7 Bxg2 38.Rxg2 h4 39.Bxh4 Rxg2+ 40.Kf3 Rxh2 41.Bxe7 Rh3+ 42.Kf2 Rb3 43.Bg5+ Kg6 44.Re7 Rxb2+ 45.Kf3 Ra8 46.Rxe6+ Kh7 0–1
5.Qb3 is playable (though perhaps not as good as 5.cxd5), but 6.Qxb6 is a mistake.
BTW, there is a nice discussion of this endgame (starting from Black's 10th move) in Shereshevsky's
Endgame Strategy.