When I was using T5HO, the ventilation was a big issue (to prevent overheating). So I had a thermostat, which turns on and off the venting fan. The venting defeats the main purpose of the tent to maintain high humidity (majority of plants in my grow tents are orchids, but I do have fair number of CPs there) without damaging the house. So I had to use an active humidifier, which was controlled by a humidistat. Actually, I was actually using ZooMed HygroTherm as a combined humidistat and thermostat. Now I use high efficiency LEDs (you will still have heat issue if you use low quality, cheap LEDs), so I can maintain the optimum temp without any venting. So, I don't need the active humidifier any more. Water catch trays at the bottom of the grow tent is enough to maintain 80%RH (when ambient could be 30%). You can see the water catch trays in the photo in
this link; one each for the top and bottom levels
You shouldn't confuse venting with circulation within the tent. I still use fairly strong circulating fans. Circulating fans are to break the micro-environment around the surface of leaves and roots. For example, near the stomata, O2 rich region could be created. This can interfere with carbon fixation especially in C3 plants (called photo respiration). Similarly for roots (of orchids), ventilation help to bring O2 closer to the roots. The circulating fans are on timer (e.g. 30min off 1 hour on etc), which I use to control the drying late of potting media.
Now, my ventilation fan never kicks in. You might be wondering if the grow tents have unfavorable environment of high O2 and low CO2 without the ventilation. I have tracked the CO2 concentration in the tent with an IRGA sensor. Indeed, during the day, CO2 level goes down a little bit. But it isn't something I would worry. The tent isn't too tight (i.e. I haven't made an effort to close down the vents, or seal the stitches). So the passive air exchange seems to be sufficient. Also, I usually opens the door once a day to check them. CO2 level quickly goes up.
I personally prefer 4'x2' footprint. With 4'x4', it is difficult to get to the plants in the back (unless you make it as a walk-in type).
With regard to the shelves, which aren't tall enough, you can probably use linear module style LEDs, which can be made as very low profile. Something like Samsung Q-series (an updated version of H-series, which I posted
here) would be best.