You say: "That brought another surprise: our galaxy probably has four, not two, spiral arms of gas and dust where stars are forming..."

We've long thought that the Milky Way had four arms (at least). The new information here is that the Milky Way is more likely a center-barred galaxy with two major arms, the other arms being demoted to minor arms. This is a pretty significant shift in the presumed shape of the MW, as we had thought previously that it looked a lot more like Andromeda, with many equal-sized arms.