Are you are talking about a 150w MH lamp and a 400w MH ballast. If so, the lamp will work, but the lamp will have a short life, because the ballast is putting too much power into the lamp (it is also dangerous). If say, you had a 150w ballast and installed a 400w lamp (MH or HPS), the ballast will have a short life (it is also dangerous), try to match both ballast and lamp with the same wattage. I used a 400w HPS lamp in a 600w HPS ballast for a month (by accident, silly mistake). It didn't do any damage, i may have been lucky. Also, believe it or not, there was no difference in the plants under the 400w lamp (600w ballast) and my other 600watters.