How to switch between four characters when a button is pressed?

Since you have 4 of them, you want to setup a round-robin that toggles the "enabled" state of each camera accordingly. So, in a script, in the Update() function, check for the 'z' key like this. I just tested this script by attaching it to an empty object I added to my scene, and I added 4 cameras to the scene. Be sure to import Collections.Generic for List or just use arrays. You also might want to toggle/disable the audio-listeners as well since Unity will complain about there being multiple.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class CamSwitcher : MonoBehaviour {
 
     // Private monobehavior members
     int currentCameraIndex;
     List<Camera> characterCameras;
     
     void Start()
     {
         characterCameras = new List<Camera>();
         /// Get the cameras from the scene using Find or as a Parameter
         Camera camera1 = GameObject.Find("Camera1").camera;
         characterCameras.Add(camera1);
         Camera camera2 = GameObject.Find("Camera2").camera;
         characterCameras.Add(camera2);
         Camera camera3 = GameObject.Find("Camera3").camera;
         characterCameras.Add(camera3);
         Camera camera4 = GameObject.Find("Camera4").camera;
         characterCameras.Add(camera4);
     }
     
     
     void Update()
     {
         if(Input.GetKeyUp(KeyCode.Z))
         {
             // Increment the camera index to the next camera in the list
             currentCameraIndex++;
             if(characterCameras.Count == currentCameraIndex)
             {
                 currentCameraIndex = 0;
             }
             
             // loop over the camera list, disabling all but the chosen index
             foreach(Camera camera in characterCameras)
             {
                 if(camera == characterCameras[currentCameraIndex])
                 {
                     camera.enabled = true;
                 }
                 else
                 {
                     camera.enabled = false;
                 }
             }
         }
     }
  }