You should still just grow it under the 400watter... if you keep them close to it they will grow like bushes, if you use the fluorescent they will grow like pinetrees... sure it costs more in electricity but the plants dont stretch as much and the lower buds grow heavier because they are able to be closer to the light since it's so bushy... trust me in the end you end up with more than you spend on the little extra electricity. I'd rather have a short bushy plant with lots of light than a 3 foot tall one with a decent top and a bunch of tiny stuff at the bottom. I did that with my first grow, i ended up with only 1 1/2 ounces off a 3 foot plant because i vegged under flourescents first... now i'm vegging its clone under a HPS and the plant is a LOT bushier... which means i'll be able to keep the HPS closer to the bottom branches too.