even if you do find/get the 4th (out of 9), how/what are you gogin to do for the other 5 channels?
presumably, the urls'f for each cam/stream's rtsp feed varies form the previous one by a diffrent in the 'channel' parameter... perhaps the NVR simply does not like to go beyond 3 or 4, for whatever reason... it may be coaxed to do so, but we'd more infop about the brand/model of nvr...