DOT 4 is a type of brake fluid commonly used in vehicles. It is designed to withstand higher temperatures than its predecessor, DOT 3, making it suitable for modern cars that require better performance under stress. DOT 4 fluid is hygroscopic, meaning it absorbs moisture over time, which can affect braking efficiency. This brake fluid is compatible with both DOT 3 and DOT 5.1 fluids, but it should not be mixed with DOT 5, which is silicone-based. Regularly checking and replacing DOT 4 fluid is essential for maintaining optimal braking performance and safety in vehicles.