Assume that a company is using an X-chart with n = 4 to monitor a process and is simultaneously using an X-chart because it is worried about the possible occurrence of some extreme observations that might not be picked up by the X-chart. Assume that the control limits for each chart were constructed using assumed values for both µ = 20 and σ = 4. If a point plots above the UCL on the X-chart, by how much does it have to exceed the UCL in order to be certain that at least one of the four subgroup observations must plot above the UCL on the X-chart?