"Quartet ST" was a commercial music program written by Kevin Cowton and Rob Povey 
for the Atari ST that was published in 1990 by Microdeal.

In the Atari environment song file names typically used a ".4v" extension and
a separate sample file then uses ".set". (When collected on the Amiga this naming
was often changed to the prefixes: "QTS." and "SMP.")

There is also a ".4q" format where the file starts with the text "QUARTET", that
then contains both the appended music(4a) and sample(set) data (1:1 from the above 
files). On the Atari this file is then sometimes additionally ICE-compressed (i.e.
file then starts with the string "ICE!").

The files collected here always use the original Atari extensions with the separate
".4v"/".set" file format (not the ".4q" variant nor the Amiga "qts." naming!).