[Python] Writing an mp4 video using python opencv

I want to capture video from a webcam and save it to an mp4 file using opencv. I found example code on stackoverflow (below) that works great. The only hitch is that I'm trying to save it as mp4, not avi. Part of what I don't get is that the 'XVID' argument passed to the FOURCC writer is supposed to be, I think, an mp4 codec (from this link). If I change the filename to 'output.mp4' it tells me that the tag is invalid, so I have to believe that the XVID codec is actually making an avi file. Is this a stupid question? How do I write to an mp4?

I have found links showing how to convert an avi to an mp4 after the fact but that seems inefficient. Seems like I should be able to do it during the initial write.

import numpy as np
import cv2

cap = cv2.VideoCapture(0)

# Define the codec and create VideoWriter object
fourcc = cv2.cv.CV_FOURCC(*'XVID')
out = cv2.VideoWriter('output.avi',fourcc, 20.0, (640,480))

while(cap.isOpened()):
ret, frame = cap.read()
if ret==True:
frame = cv2.flip(frame,0)

# write the flipped frame
out.write(frame)

cv2.imshow('frame',frame)
if cv2.waitKey(1) & 0xFF == ord('q'):
break
else:
break

# Release everything if job is finished
cap.release()
out.release()
cv2.destroyAllWindows()

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