i would say you can flower at any height but its best to wait untill the show signs of maturing, like alternating nodes and prefloweres(prob. 6-8wks). what type of light are you using? once you start to flower it will probably grow twice the size, maybe more. a four foot plant seems really large for indoors to me, but with your lsting you can keep it at a good height. your light will lose some of its power the further it has to reach. you know yield depends on a lot of things, lighting, fertilization, pot size, kind of weed, and probably some things i forgot. good luck.