Convert a string to a gameobject name

I want to use a string to reference a gameobject that I already have in my code, but the console says that you can’t convert a string to unityengine.object. I have a string that says aaaa and a gameobject that has the name aaaa.

using System.Collections.Generic;
using UnityEngine;

public class WorldRenderer : MonoBehaviour
{

    public Transform Tile;

    string seed = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaacaaabaaaaaaaa";

    public int worldHeight;
    public int worldWidth;

    private int seedSub = 0;

    public GameObject aaaa;
    public GameObject aaab;
    public GameObject aaac;

    void Start()
    {

        transform.position = new Vector3(worldWidth / 2 * -0.64f + 0.32f, worldHeight / 2 * 0.64f + 0.32f);

        for (int height = 0; height < worldHeight; height++)
        {

            transform.position = new Vector3(worldWidth / 2 * -0.64f - 0.32f, transform.position.y);
            transform.position = transform.position - new Vector3(0, 0.64f);

            for (int width = 0; width < worldWidth; width++)
            {

                seedSub = seedSub + 1;

                transform.position = transform.position + new Vector3(0.64f, 0);
                //Here's the problem VVV
                GameObject tilepos = Instantiate(seed.Substring(seedSub * 4 - 3, 4)) as GameObject;
                //Here's the problem ^^^
                tilepos.transform.position = new Vector2(transform.position.x, transform.position.y);

            }

        }

    }

    

}```

void Start() {
    var patternMatch = Dictionary<string, GameObject>(3);

    // Add each of your game objects into a collection, with their string as a key.
    patternMatch.Add("aaaa", aaaa);
    patternMatch.Add("aaab", aaab);
    patternMatch.Add("aaac", aaac);

    //...then later, inside your for loop...

        // Form the key you want to search for.
        string key = seed.Substring(seedSub * 4 - 3, 4);

        // Look for a matching entry in your dictionary, and error if missing.
        if(!patternMatch.TryGetValue(key, out GameObject prefab)) {
            Debug.LogErrorFormat("Could not find matching prefab for pattern "{0}"", key);
            continue;
        }

        // Now we can use the object we looked up.
        GameObject tile = Instantiate(prefab, 
                                      new Vector3(transform.position.x, transform.position.y), 
                                      Quaternion.Identity);

   // ...end of your loop
}