Generate a full detail guide for business owners on how to implement a 4-day work week. The guide must be a balanced and strategic framework, relevant for 2025. It should include detailed sections on: 1. The Business Case: A review of studies on productivity and employee well-being. 2. Different Models: A comparison of the 4/10 model vs. the 100-80-100 model. 3. A step-by-step guide to planning and launching a 3-month pilot program. 4. How to measure the success of the program with clear KPIs. 5. Common challenges and how to overcome them. Please include a disclaimer.