Unity4 problem - Bypassing camera renderTexture with ReadPixels but system out of memory

I made a game in Unity 4 Pro and it doesn't seem to be running in Unity 5 so I want to keep working in Unity 4. The problem is the pro trial expired and therefore I no longer have the render texture to camera feature. So I'm trying to bypass this issue by using ReadPixels instead.

What I need to do is to create a render of couple of layers from a camera and apply it as a texture onto an object over a period of time. Just to give a bit more context, it's a hinting mechanism for this point and click game, when you go over the scene with this magnifying glass, the magnifying glass displays highlighted objects that you can interact with. I have it sort of working. It takes screenshots of the layers over a period of time and displays it on the magnifying glass but then the memory runs out and Unity crashes saying:

"Fatal error! Could not allocate memory; System out of memory!... "

As you'll see in the code, I'm trying to release or reset the grabTexture because I believe this is what is causing the problem but I'm not sure of the proper way to do this.

 using UnityEngine;
 using System.Collections;
 
 public class CameraGrabScript : MonoBehaviour
 {    
     public bool grab = true;
     //public Texture2D grabTexture;
     public Renderer magglass;
     
     private void OnPostRender()
     {
         if(grab)
         {
             Debug.Log("Grabbing");
             Texture2D grabTexture = new Texture2D(Screen.width, Screen.height);
             grabTexture.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0);
             grabTexture.Apply();// mip maps
             magglass.material.mainTexture = grabTexture;
 
             grabTexture = null;
             //Resources.UnloadAsset(grabTexture); //function can only be called on Assets that are stored on disk.
             //Destroy(grabTexture);
 
             //grab = false;
 
             Debug.Log("grabTexture is null");
         
         }
 
 
     }
 }

Thank you for your help.