Why is more plants better under a 400W?

I want to grow a Super Lemon Haze in a 2 x 3.5 x 6.5 in soil using a 400w HPS. I was thinking of doing 4 plants and topping but i just thought to myself y am i doing 4 plants, the light is still the same but its spread across 4 plants. Whats the difference between the 4 plants and 2 plants in final weight?(or 1 plant for that matter).
I was also thinking about keeping it in the window till it was about 1ft tall and using a few CLF's at night to keep it at 18/6. My thinking was that the sun gives off more lumens and its cost eff.
Any ideas or comments would be much appreciated

u would be better off keeping them under lights the in the window ideal is not going to give u same light