Pyserial: File Not Found Error When Opening Port

Question

I am very new to working with hardware in Python. I recently installed pyserial to communicate with a laboratory device, and wrote some preliminary code to be sure it installed correctly:

import serial
import serial.tools.list_ports

for n in serial.tools.list_ports.comports():
    print(n)

ser = serial.Serial("COM4")
ser.close()

However, this immediately threw an error and the port could not open.

COM4 - USB to Serial Cable (COM4)
Traceback (most recent call last):

  File "<ipython-input-8-d5870cbfac06>", line 1, in <module>
    runfile('C:/Users/zach/Downloads/Research Code/CPC_read.py', wdir='C:/Users/zach/Downloads/Research Code')

  File "C:\Users\zach\Anaconda2\lib\site-packages\spyder_kernels\customize\spydercustomize.py", line 827, in runfile
    execfile(filename, namespace)

  File "C:\Users\zach\Anaconda2\lib\site-packages\spyder_kernels\customize\spydercustomize.py", line 95, in execfile
    exec(compile(scripttext, filename, 'exec'), glob, loc)

  File "C:/Users/zach/Downloads/Research Code/CPC_read.py", line 8, in <module>
    ser = serial.Serial("COM4")

  File "C:\Users\zach\Anaconda2\lib\site-packages\serial\serialwin32.py", line 31, in __init__
    super(Serial, self).__init__(*args, **kwargs)

  File "C:\Users\zach\Anaconda2\lib\site-packages\serial\serialutil.py", line 240, in __init__
    self.open()

  File "C:\Users\zach\Anaconda2\lib\site-packages\serial\serialwin32.py", line 62, in open
    raise SerialException("could not open port {!r}: {!r}".format(self.portstr, ctypes.WinError()))

SerialException: could not open port 'COM4': WindowsError(2, 'The system cannot find the file specified.')

At the top, pyserial recognizes COM4 as an available port, but it still isn't able to connect.

Most information I could find on this error with pyserial involves people using the incorrect syntax for a newer/older version of pyserial, but I have tried multiple versions of both Anaconda (2 & 3) and pyserial (2.7 & 3.4) to attempt to fix any compatibility issues, as well as a different python install altogether. This test is on pyserial 3.4 and Anaconda2/Spyder.

What file is pyserial specifically trying to find? Tracing it back to serialwin32, it appears it's generating an invalid port handle somehow.