Traceback (most recent call last):
  File "C:UserscalebDocumentsCodingPythonThe Modern Python Bootcampfirst_program.py", line 4, in <module>
    s = serial.Serial('COM4', '9600' , timeout=0)
  File "C:UserscalebAppDataLocalProgramsPythonPython37-32libsite-packagesserialserialwin32.py", line 31, in __init__
    super(Serial, self).__init__(*args, **kwargs)
  File "C:UserscalebAppDataLocalProgramsPythonPython37-32libsite-packagesserialserialutil.py", line 240, in __init__
    self.open()
  File "C:UserscalebAppDataLocalProgramsPythonPython37-32libsite-packagesserialserialwin32.py", line 62, in open
    raise SerialException("could not open port {!r}: {!r}".format(self.portstr, ctypes.WinError()))
serial.serialutil.SerialException: could not open port 'COM4': PermissionError(13, 'Access is denied.', None, 5)

import serial
import time

s = serial.Serial('COM4', '9600' , timeout=0)

while True:
    s.read()

ARDUINO CODE
void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
  Serial.write("hello worldn");
  delay(100);
}