12:15:18:20

The just added sixth chord is a 5-prime-limit, 9-odd-limit chord of just intonation which is an "ASS", or anomalous saturated suspension. In close position, it is 1-5/4-3/2-5/3, with steps 5/4-6/5-10/9-6/5. Another arrangement is 1-6/5-3/2-9/5, in which form it can be called a just minor seventh chord.