400?600?1000?

I been growing in a 4x4x6 for two years now using only a single 400 looking to upgrade in the very new future looking for advice. Im thinking 600 but is a 600 really that much brighter than a 400? I have never seen results under anything else.

I have a 600 watt in a 2x3x5 The temperatures can be a little hard to deal with in my small room. That said, my ballast is dimmable to 75% AND 50% strength. I can definitely tell the difference in power between the settings. If I were you, I would go with the 600. 1000 might be too hard to control temps...And the 600 is supposedly the best lumen/watt ratio.

I am super happy with my two 600s. I can dim them when I don't want to pay all the money ever for electricity.

Get a dim-able 1000w, I expect you will end up using it on full power if you have decent exhaust though.

600w is the standard system for a 4x4 space. If you want to use the height, you will need side lights instead of top lights. In that case, make a vert system, put your 400w up top and add LEDs or T5HO to your walls for side lights. This allows a larger area to be in ideal/bud-producing light levels. With top lights you will only have about 16" of bud producing space with needed light.

I think the 600w standard system is less than ideal, with the dim-able 1000w you can run the levels you find ideal.

While you are upgrading take a look into UVB.