DOT 3 and DOT 4 are completely compatible. DOT 4 has a higher boiling temp.
DOT 5 should never be mixed with the others. (Just for those that don't know.)

 

dot 3 and dot 4 are completely compatible. Dot 4 has a higher boiling temp.
Dot 5 should never be mixed with the others. (just for those that don't know.)

100% a+

 

I personally have used the DOT 5 (silicon) brake fluid in my GL1000 since 1989 with excellent results...firm brake pedals, no water absorbtion, no paint stains, long life. I've only changed it once (recently) since '89, & it looked as clean & purple as day one!) No corrosion or rust was found internally in the calipers, even after 20+ years. If you go this route, you must remove all traces of the DOT 3 fluid, either by rebuilding all calipers & master cylinders, or intense flushing (bleeding)...
Others have warned of dire consequences of switching to DOT 5, but I believe in it's use TOTALLY! It's much more expensive, but worth the investment of time & money..
and use DOT 5, NOT 5.1 as there is a difference there..
good luck
Carsh