Over here on the servers, I know have a script that monitors the Raspberry Pi (1 or 4) temperature and prints it when it rises:

#!/bin/bash

CA=0

CI=90000

while true; do

  C=$(</sys/class/thermal/thermal_zone0/temp)

  if [[ "$C" -gt "$CA" ]]; then CA="$C"; fi

  if [[ "$C" -lt "$CI" ]]; then CI="$C"; fi

  D=$(date)

  T=$(python3 -c "print('min: %f C | curr: %f C | max: %f C' % ($CI/1000., $C/1000., $CA/1000.))")

  echo "$D : $T"

  sleep 5s

done

Note this is not a service, just a basic shell/python script to run ad-hoc.

The RPi 4 is currently around ~50 C, with no fan.

According to sources, it can go up to 80-85 C before it starts throttling to reduce heat.