My .02 cents:
If cooling is not an issue... A 1000 watt light over a 4 x 4 in the 56" square tent would produce the most light per sq foot. This is a classic, efficient setup for 16 plants turned down at 12" - and will produce 2 lbs from most strains. Low yielders like OG, etc will still produce a pound or more in this setup.
- at the highest quality.
600 watt lights are the most efficient (most lumens per watt). Currently - there are no 600 watt MH bulbs. If you want to run both MH and HPS from the same switchable ballast - 1000W is a better choice as both MH and HPS lights are available in this format. 600W MH conversion bulbs run closer to 400W MH performance. If running only HPS this is a mute point.
If you have room for a 4 x 8 tray in the 4 x 8 tent - that would be a more efficient setup - and will likely produce more bud than the 4x4 with a single 1K light. BUT - 2 600's over a 4 x 8 has considerably less lumens per sq foot and the quality and yield may suffer as a result.
A 3 x 6 only offers 2 more sq feet than a 4 x 4 - but 2 x 600's over a 3x6 would bring the lumen to lumen rating closer to that of the 1000 over a 4x4. The extra room offered by the 3x6 may come in handy for other equimpent.