If you have ABS, you need to use the pressure system. You can use the MityVac for pre-ABS systems.
Refreshing the fluid in the resevoir probably won't do much good because it's the fluid in the lines and calipers that needs to be refreshed.
NEVER use the old method of pumping the brakes. That was found to be the major cause of damaging master cylinders.
I doubt it would hurt to mix Dot 4 fluids so long as they haven't had an opportunity to absorb moisture.