If you are using Android then you can authenticate with both versions. You can also look up the auth logic (links) from my project Moviebase.

Here an example for the WebView:

 private static final UriMatcher URI_MATCHER = new UriMatcher(UriMatcher.NO_MATCH);
    URI_MATCHER.addURI("moviebase://auth", "v3", 100);
    URI_MATCHER.addURI("moviebase://auth", "v4", 101);

    public void onCreate(@Nullable Bundle savedInstanceState) {

        // initialise webView
        webView.setWebViewClient(new WebViewClient() {
            @Override
            public boolean shouldOverrideUrlLoading(WebView view, WebResourceRequest request) {
                Uri uri = request.getUrl();
                return handleUriLoading(view, uri);
            }
        }

    }

    boolean handleUriLoading(WebView view, Uri uri) {
       switch (URI_MATCHER.match(uri)) {
        case 100: 
            // moviebase://auth/v3?request_token=[xxx]&approved=true
            // check approved parameter
            // handle request token
            // ...
            break;
        case 101:
            // request access token, account details for version 4 etc.
            break;
       }

    }