2,4D will not kill couch grass, it's a broad leaf weedkiller.

And anything that kills any sort of grass will kill all of it.

You appear to have added a bit to your original question, very confusing - yes, 2,4D is safe to use on lawns - although I believe its been withdrawn now, it was one of the components in a liquid lawn weedkiller some years ago.

Which "couch grass" do you want to control? Elymus, or Cynodon?