Python – PySerial [Error 5] Access is Denied

I am trying to write a program in Python that will loop to keep checking the serial port (COM4) and print out a message when the character "1" is read from the serial port. I want to send "1" over the serial port from an Arduino gadget upon the push of a button.

However, I get the error "[Error 5]: Access is Denied" when I try to create an instance of a serial object. (It automatically tries to open upon instantiation, which is where the error is, from what I can see from the file in the PySerial package that handles this.)

My code:

c = serial.Serial('COM4', 9600)
while True:
    signal = c.read()
    print signal
    print "running"
    time.sleep(2)
    c.flushOutput()

It never gets past the "c = serial.Serial('COM4', 9600), though. That's where the error pops up. How can I fix this?