One more test I think was overlooked, sorry - w/engine running, does the fan run when you turn on the A/C?
Also has the temp ever gone beyond R on the gauge?
When idling and hitting O, does the temp stay there and then fan not run now?
BTW, the 4.x volts across the ECT connector pins is just signal. Just means the ECU feed is ok. If you can't get into the wires to test live when connected, you may need to throw a new ECT in to know for sure.
ECT testing goes like this...
LG/Y is signal out at ECT. This goes to EEC. Backprobe it to check when engine runs and heats up. Ground black lead off meter to complete circuit to test. If meter does not auto range, be sure to use lower scale to read DC 5v and less.
The low speed fan comes on at 0.37v coming out of the ECT, at it's connector. Must be above 50° ambient
• 0.60v = 194°, below N on the gauge, stock thermostat is full open @ 192° ~ 195° depending which is installed. My SC has a 192° stat as of 2.15.19 that went in back in 2017.
• 0.53v = 200°~ 205° and N on the gauge
• 0.46 left side of O, 212~215°
• 0.42 almost straight up O ~ R, 218°?
• 0.37/.39 220°~222° low speed fan turn on, left side of R on gauge
• Fan comes on and drives it back to N +/- depending on load and ambient air temp, etc.
Notes:
- lo speed on at 222° and above / off at 214°
- hi speed on at 228° and above / off at 220° …if hi is on, it will drive temp down and shut off at 220°... lo will also still be on, pushing temp down to 214, then fan is all off.
- A/C on runs lo speed fan, not hi.
At this point,-if- there is a problem, and it's electrical, I'd bet on it being the ECT sensor. and/or harness~connector issues. If it's mechanical, I'd guess air in the system, could be a one time thing, or there could be something going on, such as headgaskets, water pump sucking air/worn out, bad thermostat, dirty radiator, etc. I'm not sure you have an issue just yet, but I'll wait for answers to the 3 questions above.
Ken