The fashionable move, supplanting 4...Bf5 and 4...Nd7.
1. e2e4  1... c7c6  2. d2d4  2... d7d5  3. b1c3  3... d5e4  4. c3e4 This was once the undisputed main line of the Caro-Kann Defense. Nowadays the advance variation (4.e5) is also very popular.