hey, have a look to Writeable HTTP Gateways which describes the usage of the writable gateway endpoints.
The writable gateway endpoint is not the same as the HTTP API (API runs on 5001 and gateway on 8080). You are getting a 404 because the path you are posting to is not handled by the gateway (but it should be by the API).
Depending on your access-control requirements, you may need to setup a reverse proxy in-between to control authentication etc. or to limit the API surface area that is exposed.