thanks for the clarification
Again, If you have DOT 3 use that but if not DOT 4 would do coz they are compatible , unlike DOT 5 which does not mix due to it being Silicon based unlike DOT 3 which is a mixture of glycols and glycol ethers. DOT 4 contains borate esters in addition to what is contained in DOT 3. and hence this makes them compatible.
But I recommend to change the whole fluid rather then mix. And do remember to bleed your brake system with new fluid at least every year. As any fluid absorbs moisture over time and this reduces the boiling point and water as we all know has a boiling point of 212 F. An example is DOT 3 fluid with 3% water content would still exhibit a boiling point of 300F. However if water got into the system, and a big globule found its way into a caliper, the water would start to boil at 212F causing a vapor lock condition and possible brake failure.