By default pgAdmin4 runs on loopback adapter, to make it run on ethernet (eth0) you need to change some of configuration options.
You need to add below config options,
DEFAULT_SERVER = '0.0.0.0'
in config_local.py (in "pgAdmin4" folder).
Now restart pgAdmin4, Now try accessing pgAdmin4 using IP address you mentioned(eg, http://172.xxx.xxx.xx:8081)
Yeah, but *don't* do that without switching SERVER_MODE = true to ensure that authentication is enabled, otherwise you're opening pgAdmin (and any saved passwords etc) to anyone on your network.
For production use, you should use Apache or similar per the instructions I just posted.