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

Capability of encoder and decoder is good for this usecase. You can check by running:

$ 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?]