The easy way for Black to keep the balance, and also the main line, is maintaining the symmetry: 4...Bd6.
If Black wants to unbalance the game and reduce the chances that the game ends in a draw, they can try 4...c5. Although this allows White to give Black an isolated d-pawn (5.dxc5), Black gets a space advantage and some initiative as a compensation.
Black can also try to go for queenside castling and unbalance the game this way: 4...Nc6, followed by development of Bc8 and Qd8.

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Wikipedia has related information at French Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.

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