The forth wire is a ground  .In the old days of analog volume controls an unearthed metal cased pot was more susceptable to mains hum pick up  .These days there are lots more noise sources which could cause grief.This noise pickup issue applies irrespective of operating voltage  .For high voltages there are safety reasons for grounding metal objects  .

The fourth wire is a ground. In the old days of analog volume controls, an unearthed metal cased pot was more susceptible to mains hum pick up. These days there are lots more noise sources which could cause grief. This noise pickup issue applies irrespective of operating voltage. For high voltages, there are safety reasons for grounding metal objects.