I looked at every courtesy lighting diagram from 95 to 2001 and didn't see any 4 wire mirrors. 3 for the standard lighted mirror and 5 for the electro-chromatic ones. They're wired to the backup lights so that when you are in reverse the auto dimming is turned off.
I looked at the 2000 Sonoma courtesy lights diagram. 3 wires for regular mirror and it's actually 7 wires for automatic day/night mirror. Orange black and white for base mirror. Light green, brown, pink, gray, orange, white and black for the automatic mirror.
What is the mirror out of?
Well shut my mouth! I looked on 2 different brands of wiring diagrams and they both showed 3 or 7. Guess I should know by now not to believe everything I read. Do like the fact the replacement connector has every wire the same color. That should make hooking it up fun.