I'm not too versed in lighting outside of HID, but I would assume 750watts of led in 4x4 would be appropriate. I use a 1k HID in a 5x5 and it's about as small a tent as I would go for this particular light. If LED's are as efficient or more efficient than HID at turning watts into lumen then I'd say you have a good amount of light for 4x4=46 watts a square foot. Then again the 50watts per square foot rule may only translate to HID. I read into LED's 5 or 6 years ago and it was overwhelmingly a negative reaction with a few suspicious successes. Seems that is changing though, which is great. The HID is wonderful, but there is something inherently brutish and archaic about it.
Do you know anything about induction lighting? I've seen some positive stuff on that, if start up money wasn't an issue, definitely something I would consider myself. They're sort of like suped up florescent tubes that are charged by electromagnets, never need to change the bulb..but for all I know you already know plenty on that subject.
Additionally, I would think if you had a sizable budget from the start, you would want to get the most powerful light that would comfortably hang in your tent, especially because you wont have to cool off an HID monster. Do they make anything bigger than a 750 watt LED rig?
I wrote all this and then did a little googlin, it seems that the led's will be equivilent to a higher wattage "normal" bulb, i.e. 1100 watts is equivalent to 1200. Considering that I would have to say personally that a 750watt LED rig would be sufficient for plants in a 4x4 area.