afaik supplementing co2 is all about maintaining ideal co2 levels. that is why people usually use a ppm meter if they want to do it right. according to the article below, 1,100 ppm during the day and 500 ppm at night is optimal. i don't think you can achieve these co2 levels simply by exchanging air. i think the key word in your post is
replacement. so the question we have to ask is, how much co2 is being replaced? subtract that amount of co2 from 1,100 and you now know how much co2 will benefit your garden.
https://www.icmag.com/ic/showthread.php?t=221893[/I][/COLOR]why is a 4'x4'x8' room too small for co2? you can supplement co2 in any room. sure, it'd probably be a waste of time if you had a 2'x2' tent or something, but this dudes got plenty of space.
@op
doing co2 right isn't cheap. you're going to want to pickup this:
http://www.controlwizardproducts.com/proddetail.php?prod=CO352
good luck