What is the purpose of the 4.7K resistor? about minimalsportvoltmeter HOT 1 OPEN

There is a voltage difference between the Arduino, which is running on 5V, while the S.Port is running on 3.3V. The resistor is there to reduce the current between the Arduino and the S.Port, so that it doesn't fry the S.Port. It isn't strictly necessary, but it is a safety precaution.

4.7K was chosen to limit the current to a maximum of around ~1mA@5V. This is a current that almost all microcontrollers can handle without difficulty.
If a lower resistor is chosen, more current flows, potentially putting the S.Port at risk. If a higher resistor is chosen, the signal might not get through.

But I reckon, you will probably be fine with anything from 1K to 10K.

from minimalsportvoltmeter.