A "4/1 ARM" is a type of adjustable-rate mortgage where the interest rate is fixed for the first four years. After this initial period, the rate adjusts annually based on market conditions. This means that your monthly payments can change each year after the first four years, potentially increasing or decreasing. This mortgage option can be appealing for those who plan to sell or refinance before the adjustable period begins. However, it's important to understand the risks, as payments may rise significantly after the fixed period ends, depending on the interest rates at that time.