DOT3 and DOT4 brake fluids should not be mixed. The two can react negatively with one another and cause corrosion of a vehicle’s brake system. They also have significantly different boiling points, with DOT4 having a higher boiling point than DOT3.
DOT4 has a minimum boiling point of 450 F when dry and 311 F when wet, whereas DOT3 has a minimum boiling point of 400 F when dry and 284 F when wet. A vehicle that uses DOT3 brake fluid can also use DOT4, but a vehicle that uses DOT4 cannot use DOT3. Using DOT3 in a DOT4 brake system can cause the brake fluid to boil, leading to brake failure. It is safer to adhere to the recommendations of the vehicle manufacturer when it comes to brake fluid.