I grew for several months with 4 LED bulbs over a 8' long shelf. They were around 18" above the shelf. It grew fine, not as even as it could have been but I just rotated the trays once a day. I would have preferred to have set up 6 lights.
Really the lights aren't that important. The plants will grow fine without any light. Hell, they actually grow faster without light because they will stretch out to look for light. If your light is too low they can get really leggy.