It al depends of your lining style. I line at about 10 - 10.5 Volts. Feel free to give the machine what it needs. Close the give all the way and open it a hint, that's the secret of a good lining with it.
I use 3.2mm for everything so with the 4mm you probably can try even higher. (11 volts)
I color ad shade around 8-9 volts.
Experiment with voltage to find your sweet spot.
Hope you enjoy the machine.