4ths are upside down 5ths.
Since 5ths are so very important in music, 4ths are equally important. Any chords that uses a 5th will have a 4ths when it is inverted, so they are used for making inversions of major and minor chords. They are used for inversions of diminished and augmented chords, which does not change their sound but does change their look on the page. In addition, they are “stacked” to make really cool chords in more modern traditional music and jazz, something that is called “quartal harmony”.