I have a cable from the FW1 release of the G50 but my transmitter is upgraded to FW2. Should I rewire the provided cable to be connected only to pin 1&3?

Pin 4 is a 10k resistor to ground. If you short pin 3 to pin 4 you effectively lower the input impedance the guitar sees to 10k ohms instead of 1M.

This will effect the sound of some guitars depending on how they are wired. The only real way to be sure is to try both and see which you prefer. You will not affect the performance of the bodypack either way.

The proper pinout is here:

http://line6.com/support/docs/DOC-1875

Shorting pin 4 to pin 3 for mic is a simplified way of indicating that typically mics like to work with lower impedance than guitars. Most classic guitar amps have  a higher impedance closer to 1M -2M.

Steve