I am using a 4 ft. set up with one plant and one natural sunlight tube. Question is how far from the TOP of the seedlings should I keep the light? I am currently 4 inches away from top of the plants.
That distance is fine. And if you want to put them closer, that’s also OK, assuming these are standard fluorescent tubes. If I can, I have them almost against the plants. Just don’t get them that close until they’ve begun to grow (are showing the true leaves emerging between the cotyledons).