This is quite the pissing match.
A single 400 in a 4x4 would indeed grow pot. Would it be the frostiest, densest, most stoney pot this side of the Pecos? Probably not.
The minimum light recommended to flower under is only 2,000 lumens per foot squared. Median range would be 5,000 or so and light saturation occurs up around 7-8,000 depending upon strain.
Yes, a 400 will work, but it's not ideal. 2 400s would be much better.
Personally, if you already have the 400, I'd drop it in the middle and supplement white COBs around the outside and in the corners to get more even lighting.
Just my 2 cents