Quartets are supposed to make do with what they have; their vocals or a very small range of instruments, and even then the instruments need to accompany the vocals and take second place, not be the driving force behind the song and the vocals becoming the second force. It’s like listening to ragtime music in hard rock form, it just doesn’t feel right.
If you want to know an example of what I mean bu this, listen to this quartet do their rendition of ‘This is Halloween’.
https://www.youtube.com/watch?v=89LtMBrj-lM
No instruments at all, just their vocal chords, yet they sounded more…complete…than the Ponytones. If there needs to be instruments, they have to take a step back and let the vocals take center stage, like here.
https://www.youtube.com/watch?v=oeEAvYcQ0RM