Truly the only reason for the 4th wire is to ensure that the sensor body actually gets a decent ground circut instead of relying on the contact through the exhaust system, or as Elio states, the sensor has an isolated element that does not actually ground through the outer housing at all.
With the all steel exhaust system and copper manifold gaskets, the need for a seperate ground is pretty much a non-issue on spiders, but more in line with some of the newer vehicles that have all kinds of goofy stuff like those flex sections and non-mettalic gasketry that all combine to sort of isolate the sensor from a really good ground.
Will it hurt to have a 4 wire sensor in there?
Not really as there's truly no such thing as too many grounds.
Is it neccisary on a spider?
No.
I too am curious as to where the ground wire would attach to the ECU as most vehicles only have the signal/voltage sensing wire headed that way while the #4 ground wire goes to a chassis or block fixture. The ground simply isn't part of the equation AFA what the ECU needs to know.
If the sensor has a good enough ground circut to allow the ECU to pick up the voltage variations via the actual signal wire, putting the ground to the ECU is not going to improve on that, nor will it alter the time it takes for the sensor to change voltages after ECU mixture adjustment. The sensor puts out it's reading at the same rate regardless, unless of course it's partially plugged up with soot/carbon and not getting a good flowthrough across the bit in the middle. (even then they are not slow on output, only slow on sensing) It wouldn't even change the time it took the signal to get from point A to point B as it would still be traveling down the same length of signal wire that it was before the ground was moved.
Using long term thinking shooting a wire from the sensor all the way back to the ECU may even impede eventually as it's one more too long a stretch of wire chasing through chassis and channel to be cut/pinched/damaged/corroded/become intermittant down the road.
The goofy part of it all is that converting from a 3 wire to 4 wire sensor will do nothing in regard to open loop performance and (IMO) have such a neglidgable impact on closed loop that it's not even worth bothering with.
