By definition all DOT 4 meets the specifications of DOT 3, it just costs a little more to make. Same for Dot 5.1, not to be confused with Dot 5. There is a chemical difference between fluids. So there is nothing special about labeling a DOT 4 fluid DOT 4/3. Some folks like me just will not buy it, most must or they would quit doing it.
Dot 4 does normally absorb moisture a little faster.
There are also not regulated by DOT additives to control foaming, lubrication and corrosion. GM brake fluid used to be better at preventing corrosion that some other brands. I change my fluid every 2 years now so that point is moot and I have no recent information.
Rod