Not all sure how much of this is true but info I so far know and go by,
DOT 4 has a higher boiling point and also has borate esters.
DOT 5 is silicone based
This is the dry boiling point minimum as I know of;
DOT 3, 205 C (401 F)
DOT 4, 230 C (446 F)
DOT 5, 260 C (500 F)
DOT 5.1, 270 C (518 F)
In Australia DOT 3 is supposed to have min 230 C boiling point.
From what I learned DOT 2 was supposed to be drum brakes then the introduction of front disk brakes DOT 3, and introduction of all wheel disks DOT 4, and DOT 5.1 late abs but what also happened is Toyota went back to DOT 3 possibly because of viscosity but haven't researched it yet exactly.
I think so far the army is one of few using DOT 5 but are thinking of going back to DOT 3. This I can't back up yet so don't take my word for it.
Having stated this for example the nulon DOT 3 has a boiling point of 260 C and states to not have adverse effect on physical properties of rubber cups and O rings.
I'm not a scientist and some manufacturers can and do make better fluids but I also found over time one will be good for a while then go slack on product.
Once I did get frustrated when I had to diagnose a fault on a car which had front vibration at 70kmh-80kmh then slightly less at 90-100. I did all usual check wheel alignment, balance tyres,check steering rack, replace two lower ball joints, then overhaul calipers and finally found one cup swelled up in a master cylinder and would block a hole which would maintain pressure on front calipers causing front wheels to fight left each other. The fluid just came due for a flush and was DOT 4 and I'm aware it's also true it can depend on car use and storage for fluid to need earlier flush.
Once I did a concentric clutch conversion on a car from ram clutches and ram clutch recommended only DOT 3 stating other brake fluids can affect the rubber components.
So my conclusion is use the recommended DOT but if necessary can mix DOT 3 and 4 only if last choice but then flush properly with correct fluid with two people not just gravity bleed because this stirs up everything in slaves and calipers and gives better indication when old fluid is flushed.