The rule of thumb for EVERY vape device you'll ever own is the same. Start low power, take a draw and bump it up to repeat. If it starts to taste funny, gets too hot or tastes burnt, back it down. Different flavors will have different sweet spots so you just have to experiment.
With the 30 watt, the lowest you can set the watts is 5w. You didn't mention what the resistance in the CE4's were and I don't remember back that far in my personal vape so start low, 5 to 6 watts and work your way up.
"Click 3 times on the fire button to switch between watts or volts."