Also relates to the aspect ratio of the sensor (or rather has conformed to it). The old 35mm film days the negative size was 36mm x 24mm hence being a ratio of 3×2 or 3/2. Hence this recognised standard being used across the variety of different full frame and smaller sensors (lost count of how many derivatives are floating about now). There is talk of the higher end Canons dropping this aspect and going to a square format to better utilise the light circle.
Hence 4/3 sensor size is 17.3×13 which is a 4×3 ratio or 4/3.
Used to be a popular format of old with 6 x 4.5 medium format cameras (still used in the majority of digital backs).
Never knew about the Video camera Tube background … interesting.