I've seen a few other posts regarding web adaptor 404 issues, but I have a feeling that our setup may be somewhat unique.
I'm running AGS 10.4.1 on our network, have a reverse proxy server setup and it is properly forwarding all other requests as expected (http://gis.cedarfalls.com). This r-proxy was setup initially without a domain name by my IT folks. As of today however, the above domain is now operational. The catch to this is we don't host cedarfalls.com, it's hosted by a vendor, but we had gis.cedarfalls.com routed to our r-proxy server. Now I'm unable to install the web adaptor. It goes through the install, but throws a 404 when going to the config page. I've attempted to replace localhost/arcgis/webadaptor with the server name, server ip's, and the domain name being forwarded to it with no luck.
Hoping someone else has a similar setup and has solved this.