A ballast typically has a transformer, in this case a step-up variety. A 400 hps usually requires 270-350v, and a 1000hps requiring much higher. The Ballast also has a capacitor of a set rating 30-75 uf depending on the bulb. The ballast also has a way of getting the bulb very excited at start-up and that is an igniter. Now when you stick a 1000w bulb in a 400w socket then transformer,cap,ign have no way of firing the bulb. My guess is that it will be dim, not start, or just plain burn shit up.

NOTE:
Now just dont go hooking up 270-350 volts to your light to skip the whole ballast thing(for those too stoned).