wow just realized i was so stoned i forgot to post the picture on my last comment lol.
As for flowering, you should be able to do it in the same box unless your trying to have a continual grow, ie. vegging one set while the other flowers. But if you just looking at start to finish in one box youll be fine, you probably wont have to make them much shorter than they would naturally grow unless you got some tall ass sativas.
I dont know how many plants your thinking about but if you only have a couple and you want to max out your space look into topping them, that will make them short and bushy. As for flowering them young, you got a pretty decent hight with 4ft, as i said before unless there just a tall strain you can probably let them veg for at least 4 weeks before putting them into flowering.