The four words are all deal with the production of light. I'm not a technician or chemist, so please check my facts for accuracy, but here's a quick explanation:

candle = obvious source of light; also "candlepower" and related terms are used to measure the strength of a lightsource

lumen = a Latin word for "light," giving us "luminous," "illuminate," etc.; it's also a measure of the perceived power of light

phosphor = a type of metal compound that emits a sustained glow when exposed to a certain energy source

fluor = a mineral group, probably also capable of glowing (and thus somehow connected to light production, as in the word "fluorescent lamp")