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DOT 4 wasn't always around. In fact, neither was DOT 3. I'm not certain the exact year but sometime in the early '80s it was specified by Volvo. The post was for a 1988 car.
As far as fluid that is approved for DOT 3-4-5. I am certain you are misinterpreting the label. It might say that it meets the boiling standard for all three but I am not at all convinced that it is pretending to be some universal brake fluid. They are complete different and non-miscible bases. The exception is an expensive, hard-to-find newer type called DOT 5.1 that is a synthetic base that is compatible with type 3 & 4. And like those, it is NOT compatible with DOT 5 silicone based fluid.
Brake fluid used to be based on vegetable oil for its compatability with natural rubber seals. Anybody with old UK or Swedish cars knows about Girling brake fluid. It's Wesson oil with dye (kidding).
A post-war shortage of natural rubber and the creation of new compounds in the 1950s meant that other fluids could be used. The properties needed for hydraulic brake use created a class of sealing materials that did not tolerate the presence of petroleum. Most fluids were based on alcohol and or glycol but no industrywide standard existed. Sometime in the mid/late-60s, a fluid based on glycol with a minimum boiling point (~ 400F/205C) for automobile brake fluid was specified by Dept of Transportation to be labeled DOT3.
Faster cars, disc brakes everywhere, ABS meant a fluid with a higher boiling point was needed. Add some boron compounds to DOT 3 (not sure if it was boric acid or not) and the boiling point goes up 40+ degrees. That is essentially what DOT 4 fluid is.
Both are miscible in water, and tend to attract it. The good thing is that the water is kept in supension and doesn't settle in one place to attack the inside of metal parts. But moisture weakens the added corrosion inhibitors. This is why 2-3 year fluid flush is recommended by many manufacturers.
DOT 5. Higher boiling point than any glycol based fluid. Silicone base. Not miscible in water. Silicone does not mix with glycol. What does this mean? Add DOT 5 to a glycol fluid and it floats on top. Moisture, instead of dispersing into the fluid, collects its droplets and falls to the lowest point in the system where it can attack the cylinder bores. In the case of disc brakes, this is also the hottest point and most likely to boil. Silicone is not compatible with all brake seal compounds and may cause them to deteriorate much like petroleum oils do.
Flushing an existing system completely is very difficult, making DOT 5 most appropriate in new or virgin brake systems only. Many manufacturers strictly warn against its use.
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