4K all the way. I have 6 4-bulb panels in my 20X30 shop. I removed the ballasts and went with single sided LED tubes. If I were to do it all over I'd go with the double ended ones.(single end = both wires into one side of the bulb, dual sided = one wire into each end), as they seem more available. So far had them for about a year with only one that has a section unlit, but not noticeable enough to change it. Nice, bright, natural light.