Which brake fluid type can mix with DOT 4?

The correct answer is that DOT 3 brake fluid can mix with DOT 4 fluid. Both DOT 3 and DOT 4 brake fluids are glycol-based. This compatibility allows for them to be mixed in situations where a complete fluid change is not feasible. When mixed, the properties of DOT 4, which generally has a higher boiling point than DOT 3, can help enhance the overall performance of the brake system, providing improved resistance to vapor lock under high temperatures.

In contrast, DOT 5 is silicone-based and is not compatible with any glycol-based fluids (including DOT 3 and DOT 4). Mixing these types can lead to brake system failure. DOT 2, while it might seem to be a possible option, is an older specification that is not commonly used or recommended due to its lower performance characteristics. DOT 6 is not standardized in the same way as the others, and as of now, there is no commercially available DOT 6 fluid that would mix with DOT 4. Therefore, DOT 3's compatibility with DOT 4 makes it the correct answer.