it will still draw 600w just the output would be only 400w.
you have still not understood
the ballast is the supply it is not the load, this important for you to grasp
The bulb is drawing the power from the ballast, the ballast in turn is drawing the power from your mains supply
the amount of power is dependent on the load (bulb)
you do not understand these basic principles, because of this, you are getting in a muddle
the ballast can only SUPPLY 600w if a bulb with low enough resistance (600w bulb) is connected
the ballast is an OPEN CIRCUIT
the ballast requires a load (bulb) to be connected to the end of it to COMPLETE THE CIRCUIT to allow current to FLOW
the amount of power is dependent on the load, not the supply
the power the ballast is rated at is the maximum amount of power it can supply before it would burn out
you could look at ohms law perhaps maybe that would help you understand,
come to think of it, they explain this stuff to kids, using the analogy of a water pump a pipe and the flow of water
look that up, that might help
peace