The 20/4/10 rule of car buying is a financial guideline that suggests making a 20% down payment, financing the car for no more than 4 years, and keeping the monthly car payment, including insurance, at no more than 10% of one's gross monthly income. This rule can be a useful tool for car buyers but other factors such as credit score, income, and personal preferences should also be considered. It is not a strict rule, but rather a starting point for anyone looking to buy a car.