Man i wouldnt even know where to start with all that. . . Do you have co2 in your 4x4? Could you post pics? Id love to check it out? How much would that cost. . . What will happen if i dont get it?
Co2 bags raise the co2 by around 200 ppm over time I believe, cost about 10-30$ each depending where you get them. Doesn't do much imo but I don't use a tent.
Co2 regulators cost around 27$ on amazon
20lb co2 tanks run about 90$ each
50lb co2 tanks cost about 270$
Where you save money with the big tank is the cost to refill. Both tanks cost basically the same amount to refill. Only 18$ each time.
So 150-320$ total to get setup and filled.
Maybe you will want to wait until you crop out a couple times before to save up, like most people do. Co2 only a supplement, meaning without other proper conditions the co2 will do almost nothing, Co2 allows the plant to take in more light and also more nutrients and in return get more yield. So if you do not have proper lighting conditions, adding co2 would do nothing. The same can be said if you are supplementing co2 but not accounting for the increased feeding, then the results would not be as noticeable, in fact the plants would even start to look unhealthy from the additional co2 but lack of nutrients required.
It really is one of the last steps to dial in your grow, and then you must rethink how much nutrients to give.
If I were to make a comparison, Plants with no co2 probably yield 1oz-2oz each. Plants grown with co2 from veg to flower can yield 4-6oz normally, and the most i've gotten was 13oz from a single plant that vegged longer than normal as a mother plant in a 15gal. pot using co2 bottle.