Hi,
This would need other users to share experience. We only have the samples of receiving RTSP stream:
Doesn't work nvv4l2decoder for decoding RTSP in gstreamer + opencv - #3 by DaneLLL
Displaying to the screen with OpenCV and GStreamer - #9 by DaneLLL
Not sure but it may work to have 4 cv2.VideoCapture() to receive the streams. Please give it a try.