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.

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

Wikipedia has related information at Nimzo-Indian Defence.