If you’re using the port 4443, you will also need to add it to the siteurl. I assume you’re using a reverse proxy (maybe nginx) in front of your Mattermost installation, right? In this case, you do not have to change the ListenAddress or the TLS* configuration options, since most likely nginx will listen on 4443 and “forward” the requests to Mattermost which is running on port 8065 then.
May I ask why you have your roundcube running on port 443 and mattermost on 4443 instead of using two subdomains like webmail.et3.dev and chat.et3.dev, both on port 443?