The much better move 4...dxc4 takes the b3 square away from the White Queen. It also threatens continuations based on ...b5, which, if allowed to succeed, would guard the extra Pawn on c4. For this reason, the most popular move is 5.a4, preventing an immediate ...b5.
Now Black can play 5...Bf5 safely. The move 5...e6 is inconsistent and lets White get a space advantage in the center with 6.e4. After 5...Bf5, White has a choice of two plans.
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The safest continuation is 6.e3 e6 7.Bxc4 Bb4 8.O-O. If Black castles 8...O-O, White can continue building a center with 9.Qe2 Nbd7 10.e4, or go after the Bishop with 9.Nh4. After 8...Nbd7, White has the same choice: 9.Qe2 Bg6 (9...O-O transposes to the previous variation) 10.e4, or 9.Nh4.
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More speculative is 6.Ne5. The obvious reason for the move is to recapture on c4, as in 6...Nbd7 7.Nxc4, for example. A less obvious reason is that the Knight move vacates f3. After 6...e6, for example, White can play 7.f3 (7.Bg5 is also possible), preparing the move e2-e4 in one step. Black can reply 7...Bb4; if now 8.e4, the play becomes double edged with 8...Bxe4 9.fxe4 Nxe4 10.Bd2 (or 10.Qf3).
Also worth noting is that 6.Nh4 Bc8 7.Nf3 Bf5 repeats the same position seen after 5...Bf5.
