double a,b,c,d,total = 0;

foreach(DataGridViewRow row in dataGridView1.Rows)
{
    foreach(DataGridViewColumn col in dataGridView1.Columns)
    {
        a = Convert.ToDouble(row.Cells[3].Value);
        b = Convert.ToDouble(row.Cells[4].Value);
        c = Convert.ToDouble(row.Cells[5].Value);
        d = Convert.ToDouble(row.Cells[6].Value);
        total = a + b + c + d;
        MessageBox.Show(total.ToString());
    }
}