|DOT 4 is one of several designations of automotive brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.
In the United States, all brake fluids must meet [http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.asp?section=571.116 Standard No. 116; Motor vehicle brake fluids]. Under this standard there are three Department of Transportation (DOT) minimum 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 fluid's performance, and if allowed to accumulate over a period of time, can drastically reduce its boiling point. In a passenger car this is usually not much of an issue as the brakes are generally not used so hard, but can be of serious concerns in racecars or motorcycles due to their much more aggressive braking. most cars produced in the U.S. use DOT 4 brake fluid.