4x4x4 would leave you about a foot headroom for the lights, 3 vertical feet should be enough to flower 4 nice plants IF you threw them in at about a foot tall, if headroom becomes a problem you could LST them.

check this out, it may give you some ideas

DIY 4oz. minimum yield grow box - Marijuana Growing
Wert Reviewed by Wert on . First indoor grow. I've grown and budded plants outdoors before and I've got some in the process as of now. I'm interested in setting up a system indoors that'll be relatively cheap cost and something moderately easy to craft. Now I'm new to this and open to ALL days. Any tips\tricks you may have to optimize the bud, please send it my way! Much thanks appreciated! I'll upload some pics of my current outdoor grows as well to share! :rastasmoke: Rating: 5