Login using REST request gives 404
I have faced the same issue and resolved it by selecting "POST" method from dropdown list.

Find the below image:-


OUTPUT:-
{
    "current_user": {
        "uid": "1",
        "roles": [
            "authenticated",
            "administrator"
        ],
        "name": "admin"
    },
    "csrf_token": "XXXXXXXXXXXX",
    "logout_token": "XXXXXXXXXXX"
}

Note: we can get this output for the first time, if we try for the second time we will get message like 


  {
      "message": "This route can only be accessed by anonymous users." }


because postman works as a browser. 
To get it work for the second time we need to remove the cookie from postman.

Kindly let me know if I am wrong.
Thanks..