4 plants seems right. If you plan on doing hydro it way be a bit tricky to find a 2X2 resevoir (and if you want 2.1X2 forget it) I am sure there are 2 foot square containers, just never seen them, may have to mold your own container, or maybe a circle with a 2 foot diameter, but then only 3 or 2 plants. With this little room and so few plants I would suggest using clones or femized seeds so you get all females with buds, and no dud men. Since you are new to growing, if you do indeed get clones, get rooted clones so you don't have to worry about them not rooting. seeds are easier to find and can be boughten off the internet. For a small space you will want a 50/50 indica sativa plant because it will not strech like a sativa and not be as bushy as an indica.
Of course you way go down the road with soil, since you will most likely use a round container, you would have to grow 3 only. I would get a 5 gallon pot, they have a diameter a little under 2 feet, and with that size you grow 3 evenly spaced plants. I know this for a fact as I have done more in less and gotten 3 ounces a plant. 4 in a 3 gallon, it can be done, but dont do it this dense as a newbie as it was a bit hard to keep them healthy and nice, but it was done well. So don't lissen to the thought that plants CANNOT be grown 2 or 3 per pot, but 1 is indeed ideal