DOT 3 and DOT 4 brake fluids are glycol-based. They're used in most modern cars and trucks, because they work with ABS brakes. DOT 4 has a higher boiling point than DOT 3. You can use DOT 4 where DOT 3 is called for, but not the reverse. Using DOT 3 in a DOT 4 brake risks boiling the brake fluid.