Hey hows it going Cant!
I can tell you that for my tent in my apartment room (4x4x6.5 as well) I use a c02 bag and it works pretty well for my space. Its called ExHale, just google it and you can order it. If you want a DIY method, you can use 2 liter bottles with yeast, water, and sugar. I used this method as well with ok results. Just be prepared to use alot of sugar and have your grow room smell like beer lol.
As far as lighting, I would suggest a 400w hps for your flower. I also try to save as much money on electricity as possible so I now veg with cfl's and only use my 400w to flower, which isnt bad because its only 12 hrs. a day. But if you have to get a smaller hps then go for it and make sure you use supplemental lighting with it. I'm pretty sure amazon has some smaller HID lights on there, but I could be wrong.
Also, no sure if you have thought about this, but even if you aren't going to ventilate you need to find a way to keep the air circulated in there. Its going to benefit you plants majorly, trust me.
Other than that, I wish you the best and happy growing! Have Fun!
Any ideas on strains/growing method yet? just curious lol