Have you tried the latest version that uses the built in server as it may work via https? This was one reason for the change as Alexa needed the streams to be https so you can follow the guides on how to do what works for Alexa.
I’m not clear what it is your trying to do, is the camera already mjpeg and your trying to turn it into h264? that would overload the pi4 in realtime. To turn h264 into mjpeg that can be done in realtime on a pi4, just not the other direction.
I am not sure what your really asking in this post, can you explain the problem and what your wanting to do and what the problem is?