Sensor in bottom of radiator is what turns the puller fan on first under normal operation. Turns it on at roughly 206F and then back off at about 203F. The switch is actually set for a much lower temp since it's in the bottom of the radiator, but that is about what the coolant temperature is at the tstat when it kicks the fans on/off.
The #4 switch is a last resort to cool the car down during an overheating situation. It kicks on the AC pusher fan (even if AC is turned off) in an attempt to cool things down, but it doesn't kick on that fan until around 230F or so.
This is what is cool about ECMlink. You can run a wire to that #4 switch, and have ECMlink ground it to trigger the relay that controls both those fans, at whatever temp you want. I actually have mine setup to just trigger just the puller fan relay, but same idea. That way, when using a lower temp tstat like the OEM 180F version, you can have the fans come on sooner for proper operation. So now I have my puller fan set to kick on at about 194, much less than what it does under stock operation. Keeps things much cooler in traffic or while stuck in the staging lanes at a dragstrip.