If you are doing any kind of electrical work, you need to turn the breaker for that circuit OFF. Always. No exceptions to this rule. Why? Because you don't know if it's grounded properly. You also do not know if it is being back-fed by another circuit. What if that switch you turn off, doesn't control what you think it does? It may turn that light off, but your plugs are on that same circuit and HOT. I've been in houses that were wired BACKWARDS. If I didn't use my voltage tester, I wouldn't be here today. If you are not a licensed electrician, turn the breaker off! 4 Amps is enough to KILL you.