Four wires generally means its a dpdt (dual pole dual through) switch. It sounds like the contacts fell out. I'm an electronics guy and have seen this may times. Since your probably switching line voltage (120vac). Check the back of your amp and note the voltage and amperage specs. check out ampwares.com, torresengineering.com, or eurotubes.com. Any switch with the same specs that fits will do. Or take the switch to your neighborhood radioshack or local guitar dealer. You may be able to find one here. Use a pair of needlenose pliers and gently tug on the lugs after making sure they aren't soldered on.

Hopes this helps