Why do seemingly reliable sources give NFPA 704 Health ratings ranging from 1-4 for elemental mercury?
I believe that the radical rage of possible hazard ratings for elemental mercury arise from different assumptions about how the mercury is handled. I can't prove this, but it makes sense when you consider the possible hazards arising from mercury use.

Properly handled liquid mercury is not particularly harmful. There are no acute risks and a competent person could drink it and handle it with little risk. In a chemical laboratory, for example, it is common to use mercury manometers to provide limited gas overpressure for supplies of nitrogen or argon. This involves bubbling gas through mercury. As long as the vapour is safely vented to a scrubber, this is not a hazard to people in the lab. 

Equally, chemists can use mercury safely with few precautions as long as they clean up any spills thoroughly. The hazard primarily comes from vapour buildup which, if it is allowed to happen, will create serious long term chronic problems.

One example of potential danger is if non-experts come into contact with mercury. Some old gas-flow meters for domestic gas supplies used mercury which could be discharged from the meter in certain rare situations, leaving droplets of mercury around the floor near the meter. This can (and has) created a great deal of hazard in a domestic environment. This hazard is magnified enormously, for example, if the householder tries to clean the mercury spill by vacuuming it up. The result is a fine spray of mercury droplets inside a warm vacuum cleaner, which vastly increases the vapour emission and may spread fine droplets widely through the house. If badly ventilated, this can be a serious chronic issue.

The two situations span a wide range of hazard. But the difference is the context: in experienced hands mercury is not very hazardous; in other hands it is potentially very dangerous.

In short, the different rating may reflect different assumptions about who is using the mercury: someone who knows what they re doing or someone who does not.