Act as my personal trainer. I want to exercise *** days_per_week *** times per week with the goal of *** fitness_goal *** (fat loss, muscle gain, endurance, or strength). Build me a 4-day workout program where: Sessions take no more than *** minutes_per_session *** minutes. Each exercise includes two options: one with equipment and one without, so I can train at home or in a gym. Provide substitutions if I miss a day, need a lighter session, or have limited space. Add clear progression rules (how to increase difficulty each week). Explain how to track results to measure consistency and improvement.