doc: Remove mention of Qt4 from build docs

Author: laanwj

doc/build-osx.md

@@ -24,8 +24,6 @@ If you want to build the disk image with `make deploy` (.dmg / optional), you ne
 
     brew install librsvg
 
-NOTE: Building with Qt4 is still supported, however, could result in a broken UI. Building with Qt5 is recommended.
-
 Berkeley DB
 -----------
 It is recommended to use Berkeley DB 4.8. If you have to build it yourself,

doc/build-unix.md

@@ -112,18 +112,13 @@ ZMQ dependencies (provides ZMQ API 4.x):
 #### Dependencies for the GUI
 
 If you want to build Bitcoin-Qt, make sure that the required packages for Qt development
-are installed. Either Qt 5 or Qt 4 are necessary to build the GUI.
-If both Qt 4 and Qt 5 are installed, Qt 5 will be used. Pass `--with-gui=qt4` to configure to choose Qt4.
+are installed. Qt 5 is necessary to build the GUI.
 To build without GUI pass `--without-gui`.
 
-To build with Qt 5 (recommended) you need the following:
+To build with Qt 5 you need the following:
 
     sudo apt-get install libqt5gui5 libqt5core5a libqt5dbus5 qttools5-dev qttools5-dev-tools libprotobuf-dev protobuf-compiler
 
-Alternatively, to build with Qt 4 you need the following:
-
-    sudo apt-get install libqt4-dev libprotobuf-dev protobuf-compiler
-
 libqrencode (optional) can be installed with:
 
     sudo apt-get install libqrencode-dev
@@ -144,7 +139,7 @@ Optional:
 
     sudo dnf install miniupnpc-devel
 
-To build with Qt 5 (recommended) you need the following:
+To build with Qt 5 you need the following:
 
     sudo dnf install qt5-qttools-devel qt5-qtbase-devel protobuf-devel
 

doc/dependencies.md

@@ -23,7 +23,7 @@ These are the dependencies currently used by Bitcoin Core. You can find instruct
 | protobuf | [2.6.3](https://github.com/google/protobuf/releases) |  | No |  |  |
 | Python (tests) |  | [3.4](https://www.python.org/downloads) |  |  |  |
 | qrencode | [3.4.4](https://fukuchi.org/works/qrencode) |  | No |  |  |
-| Qt | [5.7.1](https://download.qt.io/official_releases/qt/) | 4.7+ | No |  |  |
+| Qt | [5.7.1](https://download.qt.io/official_releases/qt/) | 5.x | No |  |  |
 | XCB |  |  |  |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk#L94) (Linux only) |
 | xkbcommon |  |  |  |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk#L93) (Linux only) |
 | ZeroMQ | [4.2.3](https://github.com/zeromq/libzmq/releases) |  | No |  |  |

src/qt/README.md

@@ -1,6 +1,6 @@
 This directory contains the BitcoinQT graphical user interface (GUI). It uses the cross-platform framework [Qt](https://www1.qt.io/developers/).
 
-The current precise version for Qt 5 is specified in [qt.mk](/depends/packages/qt.mk). Qt 4 is also supported (see [#8263](https://github.com/bitcoin/bitcoin/issues/8263)).
+The current precise version for Qt 5 is specified in [qt.mk](/depends/packages/qt.mk).
 
 ## Compile and run
 
@@ -16,7 +16,7 @@ To run:
 
 ### forms
 
-Contains [Designer UI](http://doc.qt.io/qt-5.9/designer-using-a-ui-file.html) files. They are created with [Qt Creator](#using-qt-creator-as-ide), but can be edited using any text editor.
+Contains [Designer UI](https://doc.qt.io/qt-5.9/designer-using-a-ui-file.html) files. They are created with [Qt Creator](#using-qt-creator-as-ide), but can be edited using any text editor.
 
 ### locale
 
@@ -36,7 +36,7 @@ Represents the main window of the Bitcoin UI.
 
 ### \*model.(h/cpp)
 
-The model. When it has a corresponding controller, it generally inherits from  [QAbstractTableModel](http://doc.qt.io/qt-5/qabstracttablemodel.html). Models that are used by controllers as helpers inherit from other Qt classes like [QValidator](http://doc.qt.io/qt-5/qvalidator.html).
+The model. When it has a corresponding controller, it generally inherits from  [QAbstractTableModel](https://doc.qt.io/qt-5/qabstracttablemodel.html). Models that are used by controllers as helpers inherit from other Qt classes like [QValidator](https://doc.qt.io/qt-5/qvalidator.html).
 
 ClientModel is used by the main application `bitcoingui` and several models like `peertablemodel`.
 
@@ -46,7 +46,7 @@ A controller. `:NAMEpage.cpp` generally includes `:NAMEmodel.h` and `forms/:NAME
 
 ### \*dialog.(h/cpp)
 
-Various dialogs, e.g. to open a URL. Inherit from [QDialog](http://doc.qt.io/qt-4.8/qdialog.html).
+Various dialogs, e.g. to open a URL. Inherit from [QDialog](https://doc.qt.io/qt-5/qdialog.html).
 
 ### paymentserver.(h/cpp)