There are no scaling done for 4:3 -> 16:9 resolution. If you play an old map that has 4:3 bg without widescreen support, you'll notice that none of the objects goes to the area blocked off. You can also turn on widescreen support on that map to make sure they don't scale. They just don't add unnecessary border to make it obvious that the playfield is in 4:3. (just my guess, correct me if I'm wrong)
I don't think squishing 16:9 resolution to 4:3 the same way will work well because the sides uses more space than top and bottom (scoreboards and key press). If we downscale further to avoid overlaps with scoreboard and key press, playfield will look really squished and wide, it'll look unnatural (probably going to be something like half of top and bottom unused).
4:30AM post, probably sounds stupid