4'X4' Tent, currently have 400W, how much light could be added?

I'm using a 4x4 tent and want to increase yield. Heat isn't a problem for 7 months out of the year. What do you recommend for lights?

I could add another 400 or 600W relatively easily. This would give quite a few options for customization. Then again, a 1000W would be great too.

What's best configuration?

Are you growing big, tall plants?
If so, you might want a 1000w.

If you plan on growing lots of shorter plants (need less light penetration), you can get away with a 600w. the 600w will save you on heat and $$$.

And consider a dimmable ballast, so that you can dial it down when putting in new clones/seedlings/young plants
The hydrofarm phantom 1000w and 600w's are nice.
So are quantums

I would add the 600. It is the most efficient and can be hung at a level that will keep the 400 practical whereas the 1000 would not. I use to run two 600s in a 5x5 and produce nice buds I am sure youre going to dig the upgrade. good luck and enjoy the upgrade!

even big ops go with the 600's... they just use more of them v the 1000, too strong, too expensive, too hot.. my vented 600 is touching my plants, they in love with eachother

This might sound stupid but for a set up twice your size a 76x76x76 tent do you think 2 600 watts would be enough maby throw a 150 hps for flowering? any advice?

Thanks for all the posts. I think I'll add another 600 W. That would be 1000W total and would open the door for many different layouts.
Big loc, That area sounds prime for 3 600's if you could get rid of the heat.