DOT 3 & 4 are made from Ethylene Glycol and is hydroscopic (loves water). Dot 4 has reduced attraction to water and a higher wet and dry boiling point. Because of the attraction to water they should be changed every two years on a sttreet car. Watch for a change in color which indicates the amount of water absorbed. The darker the more water. Ethylene Glycol is a conductive fluid and the conductivity increases as the water content increases. In a system with unlike metals this electrolics causes corrosion!
Dot 5 is Silicone based. to become a DOT brake fluid they have an additive to absorb a small ammount of water, about 1%, to avoid a problem with freezing or boiling. This moisture is driven off when the system heats up. Silicone is not conductive and is a better lubricant than Glycol, so less wear. Cartel, the brand I use, has not been able to tell me how DOT 5 works with Anti Loc brake systems. (I will use DOT 4 in my Impreza Sport until that question is settled) Silicone is less dense than Glycol and could possibly cause confusion in an Anti Loc system. It is true that DOT 5 has a higher wet & dry boiling point. If you are having a problen boiling fresh, quality, DOT 4 fluid (Ate, Motul, AP, etc.) then you probibly need to look at your brake system and air flow for the problem.
I have used DOT 5 very succesfully for over 25 years in many different vehicles. It stopped the problem of rapid wear and corrosion in the hydraulics on my MG's and Mini's, and just reduced wear and maintance on my BMW and Dodge tow truck.
Tom Bishop, Auto Sport Gallery, Raleigh, NC