THE IMPORTANCE OF FIXED LENGTH ITERATIONS THROUGH THE PRODUCT LIFECYCLE

Why are fixed length sprints (iterations or timeboxes) of the same duration important?
why should these sprints not exceed a 4 week timebox?
WHY 4 WEEKS?
Scrum recommends that iterations do not exceed 4 weeks. I have heard of some implementations using 6 weeks. I stick with durations of 4 weeks or less.
4 weeks of fixed scope is [...]