The 400W hps will help them to show flowering within the first 3 days, and start heavily budding within the first 2-3 weeks.
I am around 7 days into flowering in a tent with a 400WHPS (the last two under daylight due to an inspection of my smoke alarms) and I am quite please with how much a small footprint 400W HPS can produce. I will be post new pics on my journal soon if you ever want to check out another 400W HPS
Edit: Stay on top of nutrients, ph levels, pest control. Keep temperatures consistant, and you should be doing wonderfully.