The answer on StackExhange is very good, but misses the history of why 4:3 exists in the first place, which had a great deal to do with mechanical reasons. This is well explained on Wikipedia's entry on the Academy ratio[1].
The evolution of aspect ratios---4:3, Cinemascope, Anamorphic, 70mm, etc.---is fascinating. There's a constant trade-off between aesthetic and technical requirements with a lot of really cool innovations for the time.
For my money the most interesting format is anamorphic[2].
1: https://en.wikipedia.org/wiki/Academy_ratio
The evolution of aspect ratios---4:3, Cinemascope, Anamorphic, 70mm, etc.---is fascinating. There's a constant trade-off between aesthetic and technical requirements with a lot of really cool innovations for the time.
For my money the most interesting format is anamorphic[2].
2: https://en.wikipedia.org/wiki/Anamorphic_format