No it will not produce 600 watts, thats why they make a 600 watt ballast. Spend what you can here, it is a worth while investment.
It probably will light the bulb (magnetic ballast, digital i question), and there is no reason it should shorten the ballast life, the bulb however it most likely would. I believe if you did a side by side comparison on this you would find the 400 watt bulb (on 400 watt ballast) would be brighter than a 600 watt bulb on a 400 watt ballast.
You can however run a 430w hps on a 400 watt hps ballast.