With many instruments, musicians make use of a Capo or Capotraste. This is an item with which frets can be marked on a specific position. The result of which is that the key in which you play is transported to that of another.
This is a technique which is useful to know, but the Cuatro is not played with a Capo. It is possible for beginners to make use of it if you definitely want to play in a certain key without any variation, however the Cuatro makes use of all positions along the fretboard and the usage of a Capo is therefore not recommendable.
It is therefore recommendable to increase the agility in your fingers, practice a considerable amount and be able to use the bar with your index finger along with the liberation of your bottom three fingers being the middle, ring and pinky.