It is better to upgrade to latest DeepStream SDK. DeespStream 4.0.1 is not supported now.
The nvjpegdec is always chosen by decodebin automatically(decodebin), seems nvjpegdec’s priority is higher than nvv4l2decoder. So you may need to use fixed pipeline instead of dynamic pipeline(decodebin is dynamic) to use nvv4l2decoder definitely.
You need c programming skills and gstreamer(https://gstreamer.freedesktop.org/) programming skills to understand deepstream codes.