Don't get hung up on the
number of plants. It's all about the canopy space and the yield. You could fill the 4x4 with 4 plants under 1000w ... and get 1lb. That would be more than enough crop to keep you busy. Go bigger when you've got some experience under your belt.
Use the 400w to veg and the 1000w to flower. Hydro or dirt? Make sure you plan out the ventilation. Plan your air flow, Co2 and cooling if you want to grow good bud. That's absolutely crucial for success.
You can't get Co2 on your budget, so vent to the outdoors and allow fresh air to naturally replace your Co2. Use a good filter (they are worth every penny). Consider an oversized fan and a variable speed controller
http://aeroponicsnmore.com/climate-control-ventilation-c-23_5/temp-2v-fan-speed-control-variable-idle-setpoint-p-181 to adjust the cfm. That will allow you to bypass an AC until it gets summer time.
90% of the noobs put up the light, add plants ... and fail miserably. Read a lot of journals and notice that most end abruptly with cooked plants. Everyone is super proud of their vegging plants ... but watch what happens at around week 6 of 12/12 when they have crappy ventillation
I'm just sayin! Find successful grows, rinse and repeat.