Yep, you definitely caught the grow bug! Sweet! If you want to do four plants, you will need at least a 3x3, with a 4x4 being optimal. Some 4 inch fans and carbon filter are powerful enough to handle a 3x3, but chances are you need something more for a 4x4. The fan measurement you want to pay attention to is cfh. The higher the number the more air it moves, and the bigger tents it can scrub.
As far as the light, a lot of vendors say the light is 300watt, but they really mean it's equivalent to that in a different light type. Most times they tell you the true watts from the wall, and that's different, and usually much lower. Do you know the brand and model of the light?
Also, if you are getting a tent anyway, and you have the space now, I would do it before the flowering stretch, so you can let those amazing specimens do what they naturally want to do, and that's to be beasts, without the height and space restrictions.
Anyway, the best advice I ever records ever was to enjoy the grow! Have fun!