Pluck the string with your left-hand 4th finger to develop a good finger action of the 4th finger.
Many beginner methods start out with 1st, 2nd and 3rd fingers. This exercise helps incorporate the 4th finger into the hand frame at an early stage of learning. It can also improve 4th finger use so that the 4th finger is not late in descending scale passages. Make sure your left arm carries the fingers to the next string so they are ready to play in time.
Silent practice helps encourage a focus on finger action without being distracted by tuning.
Left-hand pizzicato needs flexible finger joints, which are important for 4th finger development....