Notepad4 is a Windows-native text and code editor (fork of Notepad2-mod) built on the Scintilla editing engine. It provides syntax highlighting for over 80 programming and markup languages via custom lexer definitions in `src/EditLexers/`, a file browser plugin (matepath) with Explorer-like operations, dark mode, localization (11 languages), and a full suite of editing features including regex search (bundled Boost regex library), code folding, auto-completion, and file change monitoring. The architecture is a Win32 desktop application with heavy use of Windows Shell APIs, Direct2D, and custom dialog procedures. Key entry points are `src/Notepad4.cpp` for the main editor and `matepath/src/matepath.cpp` for the browser. The build system supports x64, ARM64, AVX2, and AVX512 via Visual Studio and MinGW. While the codebase has some legacy patterns (bundled older Boost regex, a few deprecated API calls), it is well-structured, actively maintained, and includes reusable components like the `OptionSet<T>` lexer configuration system, `SplitVector` gap buffer, and SIMD-optimized graphic utilities.