Connect Arduino To Button On External Circuit Board

I have an old bingo console that has a pushbutton switch whose state I want to monitor.

Here's what I'm "really trying to accomplish": Using my Arduino UNO, I want to detect when the button switch is in its down/pushed-in state, and when the button is up/not pushed in. The button switch has four pins. Using a voltmeter, the voltage across the button switch reads ~4.8v when the button is up. The voltage reads 0v when the button switch is down. Since this button is on a circuit board in a working device, it obviously has power and is grounded.

I'm more of a software guy with rookie-level hardware experience, so my first question is this: How do I wire this button switch up to an Arduino in order to detect its state? I've seen lots of "Arduino Button" examples out there, but they all assume you're powering the button with the Arduino's 5v power pin and grounding the button to the Arduino's GND pin. I need help wiring an existing button switch to my Arduino, and any components+specs (resistors, etc.) I will need to include in order to "see" the button change state in Arduino code.

Besure the power is isolated from the mains. If not walk away. You can connect your arduino input pin via a 10K resistor to the side of the button that goes from 0 to 5V, the grounds must also be connected.

OK, so connect the button switch ground pin to the Arduino GND pin? And how do I know if the power is isolated from the mains?

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