If you could expand the list to include a fifth highly influential item, you could do worse than adding the electric bass guitar.

I assume that the first electric basses showed up in the early 1950s. By the early '60s, virtually every popular musical ensemble of any size had a bass guitarist.

The portability of the bass guitar, compared to that of a stand-up bass, facilitated the emergence of many thousands of groups and opened the door to playing bass to countless individuals.