DOT 4 is one of several designations of brake fluid denoting a particular mixture of chemicals imparting specified ranges of boiling point.
All brake fluids must meet federal standard #116. Under this standard there are three Department of Transportation (DOT) minimal specifications for brake fluid. They are DOT 3, DOT 4, and DOT 5.1.
DOT 4, like DOT 3 and DOT 5.1, is a polyethylene glycol based fluid (contrasted with DOT 5 which is silicone based). Fluids such as DOT 4 are hygroscopic and will absorb water from the atmosphere. This degrades the performance of the fluid by drastically reducing its boiling point. In a passenger car this is not much of an issue, but can be of serious concerns in racecars or motorcycles.
MINIMAL boiling points for these specifications are as follows:
Boiling Point Ranges
| Dry Boiling Point|| Wet Boiling Point|
| DOT 3 || 205Â°C (401Â°F) || 140Â°C (284Â°F)|
| DOT 4 || 230Â°C (446Â°F) || 155Â°C (311Â°F)|
| DOT 5 || 260Â°C (500Â°F) || 180Â°C (356Â°F)|
| DOT 5.1 || 270Â°C (518Â°F) || 191Â°C (375Â°F)|
One particular brand of DOT 4 brake fluid lists the following ingredients on its MSDS:
|Chemical|| CAS no || Percent|
|Triethylene glycol || 000112-27-6 ||5-25|
|Tetraethylene glycol || 000112-60-7 ||5-25|
|Dibutoxy tetraglycol || 000112-98-1 ||10-50|
|Tetraethylene glycol diethyl ether || 004353-28-0 || 10-50|
|Propane, 2-methoxy-1-(2-methoxy-1-methylethoxy)- || 089399-28-0 ||10-50|