DOT 4 has a higher boiling point than 3,any other difference ?
mak

There could be an issue with rubber parts compatibilty.
While DOT4 in general does have a higher DRY boiling point than DOT3, it is more likely to absorb moisture. So DOT4, if left in place for more than 2 years, might have a lower boiling point than 2 year old DOT3.
Unless one is extremely hard (abusive) on brakes the boiling point is academic. Follow the makers recommendations for the grade of brake to use.