To tune the F# string on your Cuatro, you’ll also need a reference pitch to compare it to. At this point, you might have already adjusted your A and D strings, so you can use any of these two as a reference.
To tune the F# string, hold down the D string at the fourth fret with your left hand. This will create a pitch that’s the same as the open string pitch.
If you pick the F# string and you hear that the sound is higher than the reference pitch, loosen the string by turning the tuning key counterclockwise. If it’s lower than the reference pitch, tighten the string by turning the tuning key clockwise. Keep adjusting and comparing until the two pitches match.
It may take a few adjustments, so be patient and take your time. When the D string is in tune, move on to the other strings and repeat the process.