How to leave new form without depending on the other? [closed]

Here is the code that opens my Form4:

Form4 form4 = new Form4();
form4.Show();
// Aqui um código gigantesco que demora aproximadamente 1 minuto para ser executado.
form4.Close();

The problem is that while Form4 is open, it is gray and static, not move or close. I understand that it gets somewhat dependent on the previous form code, how to solve this?