yea, i hear ya. i don't think doing a few soil plants would be that big a deal. get yourself a bag of soiless mix like sunshine. you're only gonna do like 4 plants in 3 gallon pots so you don't need much. no need to get all fancy or anything. just something to play around with. just a thought.
can you string the lights together so that their footprints overlap? that way maybe you could add up all of the extra space and get 1 more setup on the end. know what i mean? instead of 4 setups under 4 lights you could do 5 setups under 4 lights. sounds like you have about 10-12" extra space on every setup. if you bunched them together you could squeeze out another setup at the end. plus, when you string lights together their footprint increases because footcandles are stackable. so instead of 5' between bulbs you could have 5'6" if your lights are in a line.