Yep, better is to keep a 4:3 source in its given proportion when encoding, means 4:3.
Most 16:9 TVs beside that dynamical stretching from 4:3 to 16:9 do also got a 14:9 mode where the Pic won't be distorted but cropped "a bit" at top & bottom, so all these filtering examples of todd barry are already implementated in newer 16:9 TV sets.
Imagine you would playback such a "4:3 modded to 16:9" source image on a 4:3 afterwars using the players internal Letterboxing ---> nonsense distortion.
Then a panscanning via the player on that modded 4:3 to 16:9 material would be inevitable.