How to create a migrate and add a calculated column to a table already in the database?

I have the table evaluations that have the columns nota1, nota2, nota3, nota4 created in the database and wanted to create a media column that already calculated the media automatically, how to create this column calculate using migrate from laravel?

public function up()
    {
        Schema::create('avaliacoes', function(Blueprint $table)
        {
            $table->increments('id');           
            $table->integer('nota1')->default(0);
            $table->integer('nota2')->default(0);
            $table->integer('nota3')->default(0);
            $table->integer('nota4')->default(0);
            //criar columa media aqui = (nota1+nota2+nota3+nota4) / 4

        });
    }



nota1 | nota2 | nota3 | nota4 | // criar coluna media
 2    |    3  |     4 |     5 | // Media aqui