The main difference between DOT 3 and DOT 4 brake fluid is that DOT 4 has a higher boiling point, making it better suited for high-performance driving situations with more extreme braking temperatures, while DOT 3 is typically used in most standard vehicles and absorbs less moisture from the air, requiring less frequent fluid changes; always consult your vehicle's manual for the recommended brake fluid type. Both DOT 3 & 4 are polyethylene glycol based the primary differences are their wet & dry boiling points and their composition. DOT 3 is the most common type used in cars and trucks today. DOT 4, however, is gaining popularity due to widespread use of anti-lock braking systems and traction control, which benefit from DOT 4 fluid's lower viscosity. Never mix DOT 3 and 4 fluid type or brake efficency may suffer.