Which kilopascal is most like sunlight? Is it 4200 or is it 6000 ?
Kilopascal is a unit of pressure. Sunlight has practically zero pressure.
I think you're asking about "color temperature," in which case the increment you're looking for is Kelvin, not kilopascal. Kilopascal measures pressure. Kelvin measures temperature, and the color temperature rating on light bulbs - because that's what this is about, isn't it? - is trying to give the equivalency of a black body radiating at a certain temperature. 4200 is generally considered a "cool white" and will pretty closely resemble the light of a typical office fluorescent lamp. 6000, though it's a hotter temperature, is considered an even cooler white. Go figure the naming conventions, I don't make this crap up. Anyway, it's supposed to be a sorta-balanced representation of the light from a sunny sky, the sun and the blue sky above. That's why it's kind of heavy toward the blue end of the spectrum. NOTE: unless your lighting levels are VERY high, so-called "daylight" bulbs, the ones with the 5000-6000K color temperature, can make people look dead. DEAD. Not enough red in the spectrum. So people look terrible. In my shop where I don't have a lot of power available, I installed a couple of fixtures with one daylight bulb and one "warm white" (2700K) bulb in each. The light levels are sufficient to get work done but not huge, but the quality of the light is very balanced and does a good job of rendering colors accurately. NOTE: if you don't want people commenting on your mismatched bulbs in the fixture, 4000-4200K is actually pretty good for color rendition, and the most common rating so they're both easy to find and generally less expensive. Good luck with it.
Kelvin is the scale used to determine color of light, the photosphere of the Sun has an effective temperature of 5778 K. The Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907).
If you mean which Color Temperature, the 6000, as the temperature of the surface of the sun is about 6000 degrees K. But if daylight is really what you want, get a tube that says Daylight.