You can switch to DOT 5 too, but you need to replace the rubber parts and thoroughly clean the system out first. The dOT 5 is harmlesss to paint and such, but also much harder to find.
You should not mix the 3 and 4 either, use one or the other. If you go to the combo3/4 stuff, be sure to drain all the old stuff first. That should be replaced every two years at least anyway.
