You need 3 types of air in your room.
1, Intake,very important to control temps & air quality,also important with natural co2 levels.
2, Exhaust, equally as important as intake,exhaust helps with all of the above plus helps keep humidity at a constant,this will be more important durring flowering.
3,Circulation,a standard oscalating fan is the best method,this helps build strong plants & helps keep away issues like powdery mildew & mold,it also helps keep your entire room a constant temp,you do not want large differences in temp between the canopy & the base of the plants,it also helps with humidity issues,cant forget fukin spider mites,the easiest way to assure you dont get them is to have kick ass air flow all over the plant,all leaves should be moving all the time,a little harder than a flutter is perfect as long as the leaves are not being hurt.
Unless your living on the equator where you live it is the cool temp time of year right now,in a few months your going to have alot more heat/humidity problems than your having now if you dont regulate air flow,you need the ability to raise or lower in room temps around 10 degrees at all times with your air system,keep in mind that the very instant you connect a carbon filter temps are going to raise.
For about a $100 bill you can set up your closet to be prepared for the summer heat wave that is comming,you can take care of all the intake & exhause right from your closet door.
You'll need 2 8" inch inline fans from Home Depot which cost $35 each, then you'll need a 20" inch oscalating fan from a dollar store,they cost around $20,preferably one with a stand that you can raise when your plants get large & are in full bloom,you'll also need 2 10" register covers which are $5 each at home depot.
Cut a 7" inch hole at the bottom of the door close to one side then cut another 7" inch hole at the top of the door twords the other side,being offset helps circulate air flow.
Mount one inline fan on the bottom ofthe door sucking cold/fresh air in & mount the other inline fan on the top of the door pulling stale/hot air out,then fasten the register covers over the open holes on the outside of the door,if light escaping through the holes from the fans is a problem its an easy thing to fix,you simply build a light box around each hole from the inside of the door so no light can escape but air flow is not restricted.
Then you should place the oscalating fan in the best location to move the most fresh air through the canopy & into the lower section of the garden,after a few days or weeks on the low setting you can increase speed as the plants grow thicker & stronger,by the time your in bloom you'll be able to have it on high without hurting the plants,possible even add a 2nd oscalating fan,one at the base of the plants & one at the canopy.
Air flow is just as important as light,look at all the indoor grow masters like Seeymore buds,watch their videos,the main thing i learned from the Seeymore buds videos is that air flow & circulation is key,you cant ever have too much air flow & your yeild will greatly increase & your furure problems with heat,mold,mildew & humidity will show a sharp decline.
Edit, I forgot to answer the height question.
If all you have is a 4ft height max you'd beter start flowering s soon as you know sex & surely below 1 ft,you'll need atleast 10" inches as a buffer zone between the canopy & your light,you can grow larger & closer to the light by running a cool tube instead of a box reflector or batwing.
