The one in the rear housing that gives the gauge a signal, not the thermosensor in the waterpump housing.
I'm asking because I want to put my aftermarket temp gauge sensor there and I haven't been able to discover it by searching.
It is different from the s5's thread size which is 1/8th NPT 28?
I'm not sure on that number, but I know my s5 temp sensor didn't fit in the s4's location...

 

Yea the S5 thread matches up with 1/8 NPT. I don't know the size but all S4's are smaller (with the exception of late model JDM engines). You may be able to find an adapter, but for the best results you should consider drilling and tapping the water pump housing for this:

https://www.rx7club.com/2nd-generation-specific-1986-1992-17/megan-racing-gauges-prosport-936029/page2/

https://www.rx7club.com/2nd-generation-specific-1986-1992-17/best-place-aftermarket-coolant-temp-senor-945416/

 

I cannot tell you what thread the S4 is, but I can confirm that the S5 is 1/8th NPT