Cálculo de média usando for

media1.cs

private void btnCalcular_Click(object sender, EventArgs e)
{
  decimal[] notas = new decimal[4];

  notas[0] = txtNota0.Value;
  notas[1] = txtNota1.Value;
  notas[2] = txtNota2.Value;
  notas[3] = txtNota3.Value;

  Array.Sort(notas);

  lblMenor.Text = notas[0].ToString();
  lblMaior.Text = notas[3].ToString();

  int p = 0;
  decimal total = 0;
  for (p = 0; p < 4; p = p + 1)
  {
      total = total + notas[p];
  }

  decimal media = total / 4;
  lblMedia.Text = media.ToString();
}