Can I decode h264 4k 4096x2160 30fps from rtsp stream and restream to 1920x1080 30fps with nano?

Hi,
Capability of encoder and decoder is good for this usecase. You can check by running:
https://jell.yfish.us/media/jellyfish-120-mbps-4k-uhd-h264.mkv

$ gst-launch-1.0 filesrc location= jellyfish-120-mbps-4k-uhd-h264.mkv ! matroskademux ! h264parse ! nvv4l2decoder ! nvvidconv ! 'video/x-raw(memory:NVMM),width=1920,height=1080' ! nvv4l2h264enc ! fpsdisplaysink text-overlay=0 video-sink=fakesink sync=0 -v

Generally we set up RTSP server through test-launch. Please refer to
Jetson Nano FAQ
[Q: Is there any example of running RTSP streaming?]