The A string on a Cuatro is the lowest in pitch and is typically tuned to the pitch A4. To tune this string, you will need a reference pitch to compare it to. This can be a pitch from a tuning fork, a pitch from an electronic tuner, or the pitch of another instrument if it is already in tune.
Here’s a video with the pitch from an electronic tuner that you can use as a reference:
Next, pluck the A string with your right hand, and listen carefully to the pitch. If the pitch is higher than the reference pitch, you will need to loosen the string by turning the tuning key for the A string in a counterclockwise direction. This will cause the string to become less tight, which will lower its pitch. If the pitch is lower than the reference pitch, you will need to tighten the string by turning the tuning key in a clockwise direction. This will make the string tighter, which will raise its pitch.
Keep adjusting the pitch of the A string and comparing it to your reference pitch until they match. It may take a few adjustments to get the string in tune, so be patient and take your time. Once the A string is in tune, you can move on to the other strings on your Cuatro and repeat the process to get them in tune as well.