Because the 4000 series graphics are not supported under Windows 10.
Products That Do Not Support Windows® 10
AMD Radeon™ HD 4000 Series products and older are not certified to support Windows Display Driver Model (WDDM) 1.2 or higher and therefore, do not have driver support for Windows® 10.
The following is a list of graphics products that do not support Windows® 10:
ATI Radeon™ HD 4000 Series /ATI Mobility Radeon™ HD 4000 Series Graphics
ATI Radeon™ HD 3000 Series /ATI Mobility Radeon™ HD 3000 Series Graphics
ATI Radeon™ HD 2000 Series /ATI Mobility Radeon™ HD 2000 Series Graphics
ATI Radeon™ X1### Series/ATI Mobility Radeon™ X1### Series Graphics
ATI Radeon™ X### Series/ATI Mobility Radeon™ X### Series Graphics
ATI Radeon™ 9000 Series Graphics
ATI Radeon™ 8500 Graphics
ATI Radeon™ 7500 Graphics
ATI Radeon™ DDR Graphics
ATI Radeon™ 7000 Graphics
ATI Radeon™ 7000VE Graphics
ATI Radeon™ 7000LE Graphics
Windows® 10 will install the default Microsoft® Basic Display Driver (MBDD) for these graphics adapters. MBDD provides basic display functionality (low resolutions and color) on a single monitor but DOES NOT support:
Display scaling control
Brightness control on integrated display panels
AMD Catalyst™ Control Center
If your system is equipped with an AMD Radeon™ HD 4000 Series product or older, you may consider the following options:
Continue using the existing operating system.
If you have already upgraded to Windows® 10 and would like to revert back to the previous version of Windows, refer to this Microsoft page for instructions.
Upgrade to an AMD graphics product that provides Windows® 10 support. For information about resellers and product availability, visit: http://shop.amd.com
NOTE: Before upgrading ensure that your system meets the requirements of the graphics adapter, such as:
Motherboard with compatible graphics card slot
Adequate power supply unit to handle the complete system power requirements
Display with compatible connections