lol no its perfectly safe to use a 400watt in a 600watt ballast :thumbsup: i sometimes run my 250watt bulb in the same ballast, As long as you dont try to put a bulb of a higher wattage into it then all will be well :jointsmile:
only trouble is the heat from my MH bulb is too great even from a distance of 3ft it cooks the top of the plants and i dont have a shade with a cooler its just the normal open shade.