4.a3 is probably the most principled continuation against 3...Bb4, and it bears the name of the German grandmaster Fritz Sämisch (1896–1975). White invests a tempo and asks Black to exchange on c3. He is calculating that the benefit he will get from the bishop pair after 4...Bxc3+ 5.bxc3 is more important than the damage to his pawn structure. After the exchange it is very simple for Black to target the weakness c4 with his minor pieces with ...b6, ...Ba6, ...Nc6-a5. In addition, the move ...c5 secures the possibility of further increasing the pressure against the c4-pawn by means of ...Rc8 and ...cxd4. White, on the other hand, will try to set up a broad pawn centre with e4 and f4 and to mount an attack on the kingside. A typical Sämisch move order is, e. g., 5...c5 6.e3 0-0 7.Bd3 Nc6 8.Ne2 b6 9.e4 Ne8 10.0-0 Ba6 11.f4 f5.
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