Newbie Help with grow box plans/ideas!

I was going to try out growing for my first time I've been thinking about this for a while I was thinking about making a 4x4x5 and 1/2 feet. I live where it gets cold out like i mean cold I was gonna insulate this thing make it like a little house so im not worried about it getting cold. I was going to buy the starter pack with soil of hgtsupply.com, so I don't have to buy individually. I was wondering on a few questions. I was gonna cover the inside walls all in mylar covering for reflective purposes. But, in case for additional heat for when I have the lights off can I use an electric heater. I was thinking putting it at the bottom of the box because the simply law of heat rises. I was hoping for some opinions on if the heater is a good idea? How should i vent/circulate air in the box? will a 400watt mh/hps be enough to grow 4 plants? Will the box be high enough? I will just post these for right now, it's quite a bit but responses and opionins will grately be appreciated!

your going to need a fan venting the hood. but a electric heater when lights are off will be putting out light.(has to be completely dark)no light leaks. rule of thumb 400watts is best for 3x3 but will work for 4x4..remember youre trying to simulate the sun 10k lumens per sq ft (obviously you dont have to have this much) but you get the idea. just read a little bit more and youll have a better idea on what to do..

hope this helps

My space is in an unfinished basement, so it can get pretty chilly down there and I have a small electric heater with a thermostat that I keep in my veg room. Between that and my fans, my temps are constantly 79-81F lights on, 71-73F lights off.

Like you said, since heat rises, you can have your intake vents low in the box, and the exhaust higher. If you place the vents diagonal from each other, it'll create a nice airflow across the space and the plants.

Go to uHaul and buy a cardboard box.
Put white printing paper all inside.
Make vents and a good circulation.
And ofcourse have room for lights and your beloved one.
Hoped I Helped.

-Nice Ol' Bud

Thank you for the replies I was strongly considering 3x3. How many plants could you fit comfortabily in a 3x3 area without them getting really crowded? Also should I just have a intake at the bottom exhaust around the top and 2 to 3 small fans in the box to circulate air and I to cool the light?