the 4 by 4 area would be the most efficient use of your light as there is less square feet to cover and it will have more light reflected back to the plants wich will result in better growth there is no need to figure the lumens at different distances as the 4 by 4 room will always have the most lumens than the 4 by 5 or 5 by 5 when compared at same distance from the light.
while you may be able to get the same yield out of a larger space you are either using more plants to fill the extra space or you are using a longer veg time to fill the extra space.
you might also consider using co2 to increase your yield and once again the lumens per sq ft become important the plants in a 4 by 4 room would be able to utilize more co2 simply because they are receiving more lumens.