Background on a Quad

Hello, I wanted to create an endless background of 3840*2160. I created a quad and applied the background as texture on top of it but of course the pitcure is squeezed and I dont want any manual rescale which will break the original resolution.

Could you tell me how to create a precise endless background of 3840*2160 ? thank you

public float Speed = 0.5f;
public Renderer rend;
public Camera sceneCam;
public GameObject self;

    void Start()
        sceneCam = Camera.main;
        self = gameObject;

    void Update()
        Vector2 offest = new Vector2(Time.time * Speed, Time.time * Speed);

        rend.material.mainTextureOffset = offest;