An example of a four-split camera setup where one camera can be brought to front. UNTESTED.

SurveillanceCameraController.cs

using UnityEngine;
using System.Collections;


enum CameraIdentity
{
	None,
	One,
	Two,
	Three,
	Four
}


public SurveillanceCameraController : MonoBehaviour
{
	const float kPadding = 10.0f;
	
	
	public Camera cameraOne, cameraTwo, cameraThree, cameraFour;
	public float blendSpeed = 1.0f;
	public CameraIdentity activeCamera = CameraIdentity.None;
	
	
	CameraIdentity targetCamera;
	
	
	Rect GetViewportRect (int x, int y)
	{
		x = Mathf.Clamp (x, 0, 1);
		y = Mathf.Clamp (y, 0, 1);
		
		return new Rect (
			kPadding + (Screen.width * 0.5f + kPadding) * x,
			kPadding + (Screen.height * 0.5f + kPadding) * y,
			Screen.width * 0.5f - kPadding * 2.0f,
			Screen.height * 0.5f - kPadding * 2.0f
		);
	}
	
	
	Rect GetViewportRect ()
	{
		return new Rect (
			kPadding,
			kPadding,
			Screen.width - 2.0f * kPadding,
			Screen.height - 2.0f * kPadding
		);
	}
	
	
	IEnumerator Start ()
	{
		while (Application.isPlaying)
		{
			targetCamera = activeCamera;
			
			cameraOne.depth = targetCamera == CameraIdentity.One ? 1 : 0;
			cameraTwo.depth = targetCamera == CameraIdentity.Two ? 1 : 0;
			cameraThree.depth = targetCamera == CameraIdentity.Three ? 1 : 0;
			cameraFour.depth = targetCamera == CameraIdentity.Four ? 1 : 0;
			
			while (
				Application.isPlaying && !(
					BlendViewport (cameraOne, targetCamera == CameraIdentity.One ? GetViewportRect () : GetViewportRect (0, 0)) &&
					BlendViewport (cameraTwo, targetCamera == CameraIdentity.Two ? GetViewportRect () : GetViewportRect (1, 0)) &&
					BlendViewport (cameraThree, targetCamera == CameraIdentity.Three ? GetViewportRect () : GetViewportRect (0, 1)) &&
					BlendViewport (cameraFour, targetCamera == CameraIdentity.Four ? GetViewportRect () : GetViewportRect (1, 1))
				)
			)
			{
				yield return null;
			}
		}
	}
	
	
	bool BlendViewport (Camera camera, Rect targetViewport)
	{
		Rect pixelRect = camera.pixelRect;
		camera.pixelRect = new Rect (
			Mathf.Lerp (pixelRect.x, targetViewport.x, blendSpeed * Time.deltaTime),
			Mathf.Lerp (pixelRect.y, targetViewport.y, blendSpeed * Time.deltaTime),
			Mathf.Lerp (pixelRect.width, targetViewport.width, blendSpeed * Time.deltaTime),
			Mathf.Lerp (pixelRect.height, targetViewport.height, blendSpeed * Time.deltaTime)
		);
	}
}