In the Slow Slav White delays Nc3 and invites Black to play other than the usual Semi-Slav lines, principally by allowing development of the black light-squared bishop outside the pawn chain. Black can develop the bishop before playing e6 because there's no need to fear the exchange at d5 as when there's a knight at c3. (Note that Black also plays 4...a6 against the Slow Slav.) Black nevertheless will most often attempt to steer the game back into the more usual Meran or Qc2 lines with 4...e6, and now the only significant independent line involves White developing the knight to d2 instead of c3. So long as Black is familiar with the lines, there's not much reason for concern. In sum, the common moves against the Slow Slav are 4...e6, 4...Bf5, 4...Bg4, and 4...a6.
If Black, however, wants to try to take advantage of White's slow system, he will play 4...Bf5 or 4...Bg4. For an illustration as to why they're possible against the Slow Slav but not against the more usual Semi-Slav games, see, e.g., Walters-Poole in the second game below. Respecting the two bishop moves, Black chooses whether to interfere with White's development of the light-squared bishop to its usual d3 square (Bf5), or whether to harass the f3 knight and threaten to damage White's kingside (Bg4).
Black cannot play 4...Bf5 against mainline development.