OLE Control Extension (OCX) files are a type of dynamic-link library (DLL) used in the Microsoft Windows operating system. They are essentially ActiveX controls, which are reusable software components that can be embedded into various applications, including web browsers, desktop applications, and development environments. OCX files provide a way to add specific functionalities and interactive elements to these applications. They are often used for creating custom user interface elements, handling multimedia content, and providing access to hardware devices. The '4' in the name likely refers to a specific version or iteration of the OLE Control Extension standard. OCX controls are typically written in languages like C++ or Visual Basic and are registered with the operating system so that applications can discover and use them. Security vulnerabilities associated with OCX controls have been a concern in the past, leading to the development of newer technologies like .NET controls.