Do you honestly get the same yield if....

You do 1 plant under a 400w or 4 plants under a 400w?

I don't see how that could be posable, have a black thumb. plant killer. So it that true?

All depends on coverage area. Some strains will get 450 grams per meter squared. You could top and train to get one plant to reach one square meter of multiple colas, or you could grow multiple plants with single colas and get that poundage.