KeyCode.Mouse4

Description

Additional (fifth) mouse button.

Use this for detecting mouse button presses. The “4” mouse button is the fifth button on the user’s mouse if this additional button exists, for example, volume buttons on the mouse. Unity defines this as the "4" Mouse button, as the mouse Button numbering begins at 0.

using UnityEngine;

public class Example : MonoBehaviour
{
    void Update()
    {
        //Detect if the extra mouse button is pressed
        if (Input.GetKey(KeyCode.Mouse4))
        {
            Debug.Log("Mouse 4 ");
        }
    }
}