Well yeah, I assumed that since you were playing 4 Nc3 Bb4, it meant that the Petrosian with 4...Ba6 is in your repertoire. If you play ...Bb7 against the Petrosian, then yeah, I would recommend 4 Nc3 Bb7 as well.

Wow, it's been a while since I've thought about this line. Here's my understanding, as best as I remember.

1 d4 Nf6 2 c4 e6 3 Nf3 b6 and here:

A) 4 Nc3 Bb4 5 Bg5, and now there's nothing really wrong with 5...Bb7, except that it allows White the extra option of 6 Nd2!?, which is maybe annoying, maybe not, but it seems like it can be straight up avoided by altering the move order. Instead, after 5...h6 6 Bh4 g5 7 Bg3, I think Black should really be going 7...Ne4, because now 7...Bb4 8 Nd2 is I guess a good version for White of the Nd2 thingy, because Black is already commited to ...g5, which I guess he wouldn't normally do against Nf3-d2.

B) 4 Bg5 is another story. Well, personally I think I would play 4...Bb7 5 Nc3 h6 6 Bh4 Be7, which should equalize comfortably. I think this is what Greet recommended in his QID book (which is notable because he did also recommend 4 Nc3 Bb4). You can also play 5...Bb4 of course, transposing to 5...Bb7 and decide whether you find 6 Nd2!? annoying. There's also stuff like 5...h6 6 Bh4 Nh5!? available.