Camect will consider anything that is listening on port 554 (the default RTSP port) to be a "camera" ... That's why your NVR is considered a camera. It also happens to be the case that the url for channel 1 on your NVR is the same as the RTSP streaming url for a regular Amcrest camera, so that's why the NVR, having been detected as a camera, produces the video from channel 1 if you supply credentials.
Right now the only way to get other channels from the NVR is to explicitly add them by RTSP url. We would like to automate this for common NVRs someday, but it will probably be a while before we get to it. In your case this won't be useful anyway, as you're already getting those video stream straight from the cameras.
If your NVR has a way to show a 4-camera view in a single RTSP stream, then you could use the corresponding RTSP url to add that 4-camera view to Camect. I don't think this is a very common NVR feature though.