White assesses that an immediate 4...Bxc3 is not a threat, but also that 4. a3, provoking the capture, is not necessary (yet). Instead, 4. e3 simply clears the way for White's light-squared bishop to develop, usually to d3, where it helps control the all-important e4 square. Black has three main responses:

• 4...O-O is the most common. Since Black will never castle queenside in a Nimzo-Indian, castling kingside now is a useful but still highly flexible move.
• 4...c5 puts pressure on White's center immediately. Taking on c5 and accepting tripled isolated pawns (after ...Bxc3 bxc3) is right out of the question, and pushing d5 now simply creates a target, so White instead continues his development and leaves the tension.
• 4...b6 is the St. Petersburg Variation (not to be confused with the Leningrad Variation). This move prepares to strengthen Black's control of e4 yet further by fianchettoing a bishop to b7, though ...Ba6 is not yet out of the question.

In all three cases White has a choice between 5. Nge2 and 5. Bd3.

For explanation of theory tables, see theory table and for notation, see algebraic notation..

1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. e3

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Wikipedia has related information at Nimzo-Indian Defence