NGINX as a Layer-4 Proxy

Layer 7 has application awareness - paths, etc. Layer 4 only has networking and application port awareness.
nginx as a Layer 4 proxy is more like a "sticky session" than layer 7 - this will (likely) persist a request from a browser client to the same instance of a backend instance.
Layer 4 proxies can be used to route traffic on a server from an incoming client (browser) to a backend server based on something like the client's IP address.

Setup an Nginx Config File

There are a few subtle but critical differences between this layer-4 config and a layer-7 config:

• the http block in the layer-7 is now a stream block
• there are no location declarations in this layer-4 config. the location references paths in the layer-7 examples, and the paths are declared by the application. Because layer-4 does not "know" about paths, layer-4 cannot interpret the paths

stream{

  # register the backend services, the node apps, with this nginx instance
  upstream allbackend {
    server  127.0.0.1:2222;
    server  127.0.0.1:3333;
    server  127.0.0.1:4444;
    server  127.0.0.1:5555;
  }

  server{
    listen: 80;
      proxy_pass http://allbackend/;
    }
  }
}
events{}