It is impossible to control what can't be measured. If we had no measurements then no metric would show the effect of a change in a control parameter. The fourth level deals with collecting information about the environment. The measurements, results, and reports are all defined at this level.
A list of metrics might include number of faults both reported and fixed, lines of code, closeness to plan, resource utilization, and performance. Lines of code is a problematic metric that does not work well in many situations including composition. Many times the current progress is reported to be on plan but when the plan indicates a deliverable it is late.