What Scrum Master should do if requirement changes after four sprints, which causes Rework on work done till date?
The Manifesto for Agile Software Development says:
Welcome changing requirements, even late in development. Agile processes harness change for the customer's competitive advantage.
If there are a new set of needs which may cause changes, it would be treated as anything else - placed into the Product Backlog, refined by the Product Owner and Development Team, and ordered by the Product Owner. The Development Team will work through the revised Product Backlog, delivering the changes, reviewing them with stakeholders at the Sprint Review, and working toward a product that is released by the Product Owner.
My concern is why requirements changes were so drastic so quickly. Was the Product Owner adequately understanding and translating stakeholder needs into Product Backlog Items? Were the stakeholders actively involved in reviewing the product and providing feedback at the Sprint Reviews? It seems highly suspect, but not impossible, that several Sprints of work would be entirely invalidated so suddenly. I'd want to understand why this happened and what can be done to prevent it in the future.
What Scrum Master should do if requirement changes after four sprints, which causes Rework on work done till date?
Why did the team fail to inspect and adapt the product in the first three Sprints?