Well, you have a height of 4 ft. That is important to remember. Now, you didn't say what kind of light you will be using is flowering, as that light will probably hanging from the 4' ceiling. Then you will want the plants to be a certain distance from that light. In this case, let just assume the light takes 6 inches of ceiling height, and you want the top of the plant to be no closer than 6". So now you you know you have only 3 ft for your plants.
So when a plant first goes into flowering, it will grow 2 to 3 times its size by the time it expires. So I think (in this case) I would start the flowering process when the girls reach about a foot.

keylime