The inside diameter of washers produced on an assembly line has an upper specification limit of 80 micrometers and a lower specification limit of 20 micro-meters. When the process is stable, 95% of the washers produced have a diameter between 26 and 74 micrometers, with a mean of 50. Assuming the process is stable, construct a 95% confidence interval for the average diameter of the next 4 washers produced.