I know its not any help, but that should be way way bigger at 3-4 months... It seems like you have had some bad luck and I really hope you keep trying and don't give up! Maybe try some CFL supplemental lighting? I would imagine you wouldn't need it for 1 plant under a 400w HPS, but maybe the problem is that you are running it in too big of a space and there is not any light reflecting back onto the plant. Is it possible for you to make some sort of box, or even some walls to put around the plant that could harness your light more effectivly?