Unreal Engine 4 GUI like this: https://pcper.com/2012/06/e3-12-unreal-engine-4-what-you-and-we-missed-at-gdc/
Unreal Engine 3 use wxwidgets，but in UE4 it looks like .NET Framework WPF. The GUI in UE4 is fantastic.
P.S. Microsoft Expression Studio 4 has the same style.
it is not wxwidgets and wpf. it is a new gui framework which is developed by unreal engine 4 team and it named "Slate UI Framework". this framework used for editor and game twice, so it is a cross platform framework and use hardware accerlation, like unity3D editor. if you use some software for example Fraps, it will tell you the full screen is one frame buffer.
The Unreal Engine 3, at least later builds of it are slowly replacing wxWidgets with WPF which you can tell by looking at some of the .NET .dll files and also by the look of the Content Browser, color picker etcetera.
It would make sense for the Unreal Engine 4 to continue that trend.
The UE4 Editor utilizes the Slate UI Framework for its widgets.
They can be inspected via the Widget Reflector tool, which allows selecting a widget currently present on the editor screen and then finding and inspecting, among other aspects, its source code, like so:
In UE4, go to Window > Developer Tools > Widget Reflector
In the Widget Reflector window, click Pick Hit-Testable Widgets or Pick Painted Widget
Move the mouse over any widget on the editor window and press ESC to stop
Notice that the widget and its corresponding CPP file will be highlighted in the Widget Reflector
On a default installation of UE4, the base folder containing Slate UI widgets should be:
C:\Program Files\Epic Games\UE_4.26\Engine\Source\Runtime\Slate\
For instance, the implementation of the SBox widget can be found at:
C:\Program Files\Epic Games\UE_4.26\Engine\Source\Runtime\Slate\Private\Widgets\Layout\SBox.cpp
Aside from being utilized to build the UE4 Editor itself, the Slate UI Framework can also be utilized in games/content created with it.
This document provides an overview of the framework by its creator.
