okay, my master clutch cylinder is leaking (i checked near teh clutch)

the actual container, in the top right hand side of teh engine bay....is pretty much close to empty, if not fully empty....

i know it says to fill up with either with DOT 3 OR DOT4....which one should I use?
does it make a difference?
what happens if i orginally had dot3 and now put dot 4? mixing the dot3 and dot4...does that do anything?