I'm replacing an old outlet. Why are all 4 wires are hot?
When testing circuits like this, you really need a meter, not a wonder stick. The voltage testers indicate the presence of an electric field. In a box like the one you have there is an electric field and it can spread to the un energized wires. The tester only indicate the presence of voltage, not how much. Meters will tell you the actual voltage on the lines.