$req->header('x-auth-token' => 'Token e45ttkfksj48sdfj4jd9abcdd');
[download]

Are you sure that the other side wants the header name all lowercase?

$req->content($filename);
[download]

This sends the filename, not the file content. Is that what you want?

Also, if the server sends a 404, are you really, really sure that you're using the correct URL? Your URL ends with a slash, maybe you should leave that out? What other ways do you have to verify the correctness of the URL you use?