Dot 3, 4, and 5.1 are all interchangeable. The difference between them is the boiling point with 3 being the lowest and 5.1 the highest. If you mix DOT 3 with 4 it lowers DOT 4 boiling point to that closer to DOT 3. Whatever you do so not mix DOT 5 with the others because it is silicone based (I believe) and they are not compatible at all. That said it would probably be preferable to completely replace the fluid with new fresh DOT 4. Hope that helps.
2001 M900Sie - Mine
2006 Buell Blast - Wife
1971 Yamaha R5-350 - Work in Progress
1971 R5 #2 patiently waiting in the shed