So, working with 4Km2... There are 13 million square kilometers of arible land on earth: https://en.wikipedia.org/wiki/Agricultural_land . If the virtical farm is strung up with 400 meters of 10cm layers, that makes 4000 layers. That would be 16 000 square kilometers of farming area, or a bit over 0.1% of all arible land. I think that with modern vertical farming, using a 24 hour year around light source, the farming surface can produce several times the avarage arible land production. Lets say 4 times... That would make almost 0.5% of the world farm production in just one tiny area. So it would take just 200 such constructions to replace tradtional farming entirely...