A 400W would be plenty for up to 5 or 6 plants in a 4 month grow, 3 or 4 if growing them bigger than most do in an indoor grow. Keep in mind too when planting seeds to double the amount of seeds for amount of females wanted. IOW, if you want 3 plants, plant at least 6 seeds to allow for males and those that "fail to thrive".
I started with 1000W lights but switched down to several 400's because it covers an area better using more smaller lights than one big one and I am a firm believer in these 400's! They're much more versitile and easier to move when needed. Be sure to get a "remote ballast" if you haven't already bought one. I also recommend getting a switchable ballast. They cost a few dollars more at first but cheaper in the long run because you don't have to buy conversion bulbs and the remote ballast keeps a lot of the heat far away from the grow area, a separate room in my case.