Making Serial Port object as a property of another class gives an error

Hi,

I've a class named 'Task' which has one property named 'serialPort' as shown in the code below:

    classdef Task < handle
      properties 
  serialPort;
      end
      methods
        function obj = Task(varargin)
          oldSerial = instrfind('Port', 'COM4');
    if (~isempty(oldSerial))
      delete(oldSerial)
    end
      
    obj.serialPort = serial('COM4');
    obj.serialPort.BaudRate = 115200;
    size_data = 1000;
    obj.serialPort.BytesAvailableFcn = {@store_plot_data,size_data};
    obj.serialPort.BytesAvailableFcnCount = size_data;
    obj.serialPort.InputBufferSize = 30000;
    obj.serialPort.BytesAvailableFcnMode = 'byte';
          fopen(obj.serialPort);
        end
  end

In the constructor I'm assigning the properties of the serial port object, but on executing this gives an error

 "Open failed: Port: COM4 is not available. Available ports: COM1, COM3. Use INSTRFIND to determine if other instrument objects are connected to the requested device".

I have checked that COM4 port is available during the creation of the Task object but still it throws this error whereas I don't have port COM1 and COM3. Can anyone please suggest me where am I going wrong ?

Thanks,