The pinky is one of the most important fingers when playing the Cuatro. This finger allows you to sing with the instrument and it is typically the most frequently free finger when marking chords. What Cheo Hurtado demonstrates in this video is how you can make use of the pinky by adding notes to chords with it. He will take the position of A7 and move it all along the fretboard.
He then gives a slight demonstration on how this can sound with a Pajarillo in G Minor.