fix 4.x links

doc/tutorials/introduction/android_binary_package/O4A_SDK.markdown

@@ -82,7 +82,7 @@ The structure of package contents looks as follows:
 
 -   `doc` folder contains various OpenCV documentation in PDF format. It's also available online at
     <http://docs.opencv.org>.
-    @note The most recent docs (nightly build) are at <http://docs.opencv.org/master>. Generally, it's more
+    @note The most recent docs (nightly build) are at <http://docs.opencv.org/4.x>. Generally, it's more
     up-to-date, but can refer to not-yet-released functionality.
     @todo I'm not sure that this is the best place to talk about OpenCV Manager
 

doc/tutorials/introduction/android_binary_package/android_ocl_intro.markdown

@@ -28,7 +28,7 @@ If you need help with anything of the above, you may refer to our @ref tutorial_
 This tutorial also assumes you have an Android operated device with OpenCL enabled.
 
 The related source code is located within OpenCV samples at
-[opencv/samples/android/tutorial-4-opencl](https://github.com/opencv/opencv/tree/master/samples/android/tutorial-4-opencl/) directory.
+[opencv/samples/android/tutorial-4-opencl](https://github.com/opencv/opencv/tree/4.x/samples/android/tutorial-4-opencl/) directory.
 
 Preface
 -------
@@ -255,7 +255,7 @@ As you can see, inheritors for `Camera` and `Camera2` APIs should implement the
 @endcode
 
 Let's leave the details of their implementation beyond of this tutorial, please refer the
-[source code](https://github.com/opencv/opencv/tree/master/samples/android/tutorial-4-opencl/) to see them.
+[source code](https://github.com/opencv/opencv/tree/4.x/samples/android/tutorial-4-opencl/) to see them.
 
 Preview Frames modification
 ---------------------------

doc/tutorials/introduction/clojure_dev_intro/clojure_dev_intro.markdown

@@ -313,7 +313,7 @@ Then you can start interacting with OpenCV by just referencing the fully qualifi
 classes.
 
 @note
-[Here](https://docs.opencv.org/master/javadoc/index.html) you can find the full OpenCV Java API.
+[Here](https://docs.opencv.org/4.x/javadoc/index.html) you can find the full OpenCV Java API.
 
 @code{.clojure}
 user=> (org.opencv.core.Point. 0 0)

doc/tutorials/introduction/config_reference/config_reference.markdown

@@ -339,7 +339,7 @@ Integration with [FFmpeg](https://en.wikipedia.org/wiki/FFmpeg) library for deco
 - _swscale_
 - _avresample_ (optional)
 
-Exception is Windows platform where a prebuilt [plugin library containing FFmpeg](https://github.com/opencv/opencv_3rdparty/tree/ffmpeg/master) will be downloaded during a configuration stage and copied to the `bin` folder with all produced libraries.
+Exception is Windows platform where a prebuilt [plugin library containing FFmpeg](https://github.com/opencv/opencv_3rdparty/tree/ffmpeg/4.x) will be downloaded during a configuration stage and copied to the `bin` folder with all produced libraries.
 
 @note [Libav](https://en.wikipedia.org/wiki/Libav) library can be used instead of FFmpeg, but this combination is not actively supported.
 

doc/tutorials/introduction/display_image/display_image.markdown

@@ -26,7 +26,7 @@ Source Code
 
 @add_toggle_cpp
 -   **Downloadable code**: Click
-    [here](https://github.com/opencv/opencv/tree/master/samples/cpp/tutorial_code/introduction/display_image/display_image.cpp)
+    [here](https://github.com/opencv/opencv/tree/4.x/samples/cpp/tutorial_code/introduction/display_image/display_image.cpp)
 
 -   **Code at glance:**
     @include samples/cpp/tutorial_code/introduction/display_image/display_image.cpp
@@ -34,7 +34,7 @@ Source Code
 
 @add_toggle_python
 -   **Downloadable code**: Click
-    [here](https://github.com/opencv/opencv/tree/master/samples/python/tutorial_code/introduction/display_image/display_image.py)
+    [here](https://github.com/opencv/opencv/tree/4.x/samples/python/tutorial_code/introduction/display_image/display_image.py)
 
 -   **Code at glance:**
     @include samples/python/tutorial_code/introduction/display_image/display_image.py

doc/tutorials/introduction/windows_visual_studio_opencv/windows_visual_studio_opencv.markdown

@@ -204,7 +204,7 @@ Test it!
 --------
 
 Now to try this out download our little test [source code
-](https://github.com/opencv/opencv/tree/master/samples/cpp/tutorial_code/introduction/windows_visual_studio_opencv/introduction_windows_vs.cpp)
+](https://github.com/opencv/opencv/tree/4.x/samples/cpp/tutorial_code/introduction/windows_visual_studio_opencv/introduction_windows_vs.cpp)
 or get it from the sample code folder of the OpenCV sources. Add this to your project and build it.
 Here's its content:
 
@@ -220,7 +220,7 @@ the *IDE* the console window will not close once finished. It will wait for a ke
 This is important to remember when you code inside the code open and save commands. Your resources
 will be saved ( and queried for at opening!!!) relatively to your working directory. This is unless
 you give a full, explicit path as a parameter for the I/O functions. In the code above we open [this
-OpenCV logo](https://github.com/opencv/opencv/tree/master/samples/data/opencv-logo.png). Before starting up the application,
+OpenCV logo](https://github.com/opencv/opencv/tree/4.x/samples/data/opencv-logo.png). Before starting up the application,
 make sure you place
 the image file in your current working directory. Modify the image file name inside the code to try
 it out on other images too. Run it and voil á: