When I try to pull video only 2 out of 4 arlo cameras showing videos in the library.

Try following-camera camera setting:

• Make sure that there is no glass in front of the camera, otherwise cameras won't be able to detect the motion
• Try increasing the motion detection
• Try to remove the cameras and sync again

In order to fix this issue, there are few steps that we need to follow:-

• Make sure that other two cameras are not showing offline.
• Check if motion sensor  of your camera is working or not(try to wave your hand in front of the camera and check if its activating the motion sensor)
• Try removing the batteries from the cameras and reinsert it back and then check if its working
• Try to reSync the cameras

If your camera's motion sensor is working and still its not showing video in the library then in that case you need to reconfigure your base station again