SOME(5.1) may well be higher but most are around 270C or so, most dot 4 are higher boiling points than dot 5.1 dot 4 typically being 315-330C boiling point DRY, wet it would be nearer 200C, for a road/race dot 4 fluid, it is considered "wet" when it has more than 2% moisture content.
The reason dot 5.1 don't need high boiling points is because they are a cold climate fluid developed for systems such as esp where improved low temp viscosity was needed.
tabetha