Hi! The plant looks nice and healthy, so thats good.
As for your set up... I personaly wouldnt dare to mix electricity with cardboard, as this material burns easily due to its structure and low density. I know that some people use them successfully, but if anything goes wrong it will catch fire very fast.
I would invest in a small closet, it has more structural integrity and it doesnt catch fire quite as easily, some particle boards are even fire retardant.
As for things you can change easily... I would line the walls (and ceiling) with some sort of reflective material. Even alu foil is better at reflecting light than brown cardboard. You want to reflecte as much light as you can to where you want it to go - the plants.
Since you are allready using cardbord I think lining the walls with plain white paper would be better then alu foil. Or just invest in some mylar, you dont need a lot of it.
In the spirit of light efficiency I would also rearange the lights, by putting 4 bulbs above and 4 bulbs on the sides.
I would also position the side bulbs vertically and the top ones horizontaly.
A reflector for each bulb would also be very useful. Even a simple reflector made from thin sheet metal painted matt white would be better then none at all.
You can easily constuct it yourself.
If this is just a veg. box than the light spectrum is fine, but for flowering you should use warmer light... 2700K - 3000K or warm wihite.
You have plenty of root space with a 5gal container. Which is a good thing, but with such a volume of substrate you must be extra careful not to over water.
Watering every 4 days seems too often to me, if you saturate your substrate it should take at least a week before that kind of wolume (with just one small plant) dries up enough to be watered again. I watter my 1gal pots only every 3 days, judging by their dry vs saturated weight.
About the nutrients. Dont over do it. Again with such a large volume of substrate for just one plant, it should be fine for quite a while before needing any suppliments.
Be sparse with nutrients and water.
Over fertilization and over watering are the most common mistakes, avoid them. This plants thirve on a lot less than one would think.
And its a lot easier to solve under fert. or under watering problems then the other way around.
You say you have a fan blowing... You should also take care of air exchange, since in small enclosed space the plants will quickly use up CO2 plus there is also the heat and humidity buid up to keep in mind.
As for pruning, I would do little to none with just one plant. There are different opinions, but removing fan leaves is not a good idea in my book. To me it seems obvious. Its like removing solar panels and then expecting to get more juice. Leaves are plants solar panels, and once the plant is flowering it is investing most of its energy in to the flowers (and seeds) so even if the fan leaves are blocking flowers, most of the energy they produce goes to the flowers. And the plant also redirects sugars from older growth to new growth, and only then it drops the old used up leaves, once it sucks them dry, dont deprive it of that. So if a leaf doesnt come off by gently pulling on it, leave it be.
You could fim or top the plant to make it bushier and then train it for max. light exposure. With light coming from the sides and above, you also dont need to remove lover branches
And again dont be to overenthusiastic with fertilization and watering.
So thats it from me and whatever you do, I wish you successfull and safe cannabis cultivation. Cheers!
