The 4000K and 5000K are referring to the Kelvin , or the correlated color temperature of the light.
Here is a copy of the spec sheet: http://www.e-conolight.com/pdf/SpecSheets/eco_spc_hl_bay_hte18_series.pdf. On the second page there is a diagram on the second page comparing the color temperature. The 4000K will be a "white" light, whereas, the 5000K will tend to appear cooler, possibly with bluer tones to the hue of the light.
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Date published: 2017-12-14