communication error c # and Arduino

SerialPort serial = new SerialPort("COM4", 9600);
        Timer timer = new Timer();
        public Form1()
        {
            InitializeComponent();
            timer.Interval=5;
            timer.Tick += new EventHandler(aktivuj);

        }
        public void aktivuj(object sender,EventArgs e)
        {
            string s;
            Invalidate();
            serial.Open();
            serial.Write(textBox1.Text.ToString());
            s = serial.ReadLine();
            serial.Close();
            label1.Text = s;
        }