i personally don't like hps for veg. plants like a difeerent spectrum for veg. i would recommend getting a conversion bulb and use halide for veg. i veg with 8 4ft agrosun fullspectrum flouros and have excellent results. you can get a conversion bulb for your 400hps ballast rather inexpensive here are a few to look at:
http://www.plantlightinghydroponics....oducts_id=1760
http://www.wormsway.com/detail.asp?sku=SMH150

hps will work for veg. many use it without problems. imo it is much better to use halide or flouro.