Short answer
DOT 4 is a glycol-based brake fluid grade under FMVSS 116 with minimum boiling points of 230 °C (446 °F) dry and 155 °C (311 °F) wet. Most DOT 4 fluids add borate esters to the glycol-ether base, which react with absorbed water and help hold up the wet boiling point. It is the factory specification for many European and Asian vehicles and a growing share of domestic ones.
How DOT 4 differs from DOT 3
Both grades are glycol-ether fluids and both are hygroscopic. DOT 4’s higher minimums come largely from borate ester chemistry. The esters react with moisture that enters the system, so the fluid tolerates water somewhat better before its boiling point falls toward the wet minimum.
FMVSS 116 allows DOT 4 to be somewhat thicker in the cold than DOT 3: the maximum at minus 40 °C is 1,800 mm²/s versus 1,500 mm²/s for DOT 3. That is why fluid makers developed low-viscosity DOT 4 versions.
Low-viscosity and racing DOT 4 fluids
Some manufacturers call for a low-viscosity DOT 4, often labeled DOT 4 LV or ISO 4925 Class 6, because modern ABS, traction and stability modules pump fluid through very small passages and respond faster with thinner cold fluid. If the manual names a low-viscosity fluid, use one labeled that way.
At the other end, racing DOT 4 fluids are formulated with dry boiling points well above the 230 °C minimum for track use. They still meet the DOT 4 grade, but high-boiling formulas are often changed more often because their wet boiling point can drop quickly once moisture gets in.
Compatibility and substitution
DOT 4 mixes with DOT 3 and DOT 5.1, and FMVSS 116 includes a compatibility test so glycol-based grades do not separate. Mixing does, however, lower the blend toward the weaker fluid’s properties, so a full flush is the cleaner way to change grades.
DOT 4 must not be mixed with DOT 5 silicone fluid. If a system has had DOT 5 in it, consult the vehicle or brake manufacturer before switching.
Reading the label
Every FMVSS 116 container shows the grade and the wet boiling point. Two bottles that both say DOT 4 can have quite different actual boiling points, so the printed figures are the useful comparison when choosing among fluids that meet the grade your vehicle calls for.
Frequently asked questions
Can I put DOT 4 in a car that calls for DOT 3?
Many manufacturers permit it because DOT 4 minimums are higher, but check the owner’s manual first. Do not put DOT 3 into a system that specifies DOT 4.
Is DOT 4 used in motorcycles?
Many motorcycles specify DOT 4, often marked on the master cylinder cap. Follow the cap or the service manual, since some older models use DOT 5 silicone instead.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].