It's hard to avoid dullness if white wants it. If 4...Bf5 is not to your liking, then 4...Bg4 is an equally viable alternative.

Larry Kaufman in his book "The Chess Advantage in Black and White" recommends playing the Semi-Slav via the ...e6 move-order. After 1 d5 d5 2 c4 e6 3 Nf3 Nf6, he doesn't consider the move 4 Bg5, which I know is recommended in Richard Pallier's book "Play 1 d4". I don't see how Black can transpose to the Semi-Slav here. Do you know anyway Black can totranspose to the Semi-Slav here? I was thinking 4...c5 may be an ok alternative if you can't transpose to the Semi-Slav. What does Pallier think about this move in his book?

In David Vigoritio's book "Play the Semi-Slav", he recommends playing the Semi-Slav via the ...c6 move-order. After 1 d4 d5 2 c4 c6 3 Nf3 Nf6 4 e3, he recommends 4...Bf5 because 4...e6 allows 5 Bd3 and he thinks Black's best is to transpose to the dull main line of the Queen's Gambit Accepted with 5...dxc4 6 Bxc4 c5 and he obviously didn't want to cover that in a Semi-Slav book. I would prefer to play 4...e6, if Black has a sound and ambitious way to meet 5 Bd3, over the more dull 4...Bf5 which I think should be left to more dull Slav Players. Do you know of any sound and ambitious way to meet 5 Bd3?