Whatever you set the watts to the
mod will automatically have the correct volts for the resistance the coil is.
Same if you set the volts.
If using a 1.5Ω coil and set it to 8 watts that will be 3.46 volts.
Put the watts to 10 and the volts will be 3.87 volts.
If put 4v on 1.5Ω and the mod only has a 10 watt limit then its probably firing it at 10 watts or 3.87 volts.