The 5th wire you're thinking of is probably a reset line, which some AT keyboards used to have.
That pin is not used on the Amiga.
Everything is sent via synchronous serial data.
The 4 wires are +5V, ground, data and clock.
Pinouts and some more information here:
http://amiga.serveftp.net/HardwareBook/connector/index.htmlLook in the keyboard connector section.
