even a 400w doesn't sound to bad for a 4x4. i seen someone pull off 11 ounces pretty easy witha 400w setup and not even vscrog.
but a 600w would be awsome.
it should work out as long as a fan is blowing on the bulb.
11oz sounds pretty normal to me for a 400W? Never tried one. I'm normally pulling 15+ oz every harvest with my 600W.
The reason for going vertical would be hoping for even better yields than that I think? I'm sure a 400W would be great for a 4x4 vertical though - the canopy area after the plants have grown towards the light might make for a better g/W yield with a smaller light - dunno?
On the other hand - lm/W is much better with the 600W.