The HD4850 does not meet Windows 10 requirements, there are no drivers available for it.
I have everything working. Installed Windows 10 64 (clean install) then installed the driver 13.4 beta for issued Windows 8 compatibility. All work settings including Catalyst
The HD4XXX series graphics products must be installed using display driver version: 8.970.100.9001. This driver is provided as a courtesy and only available via Windows Update. Please enable Windows Update and allow it to detect and install the appropriate driver.
NOTE: ATI Radeon™ HD 4000/ATI Mobility Radeon™ HD 4000, ATI Radeon™ HD 3000/ATI Mobility Radeon™ HD 3000, and ATI Radeon™ HD 2000/ATI Mobility Radeon™ HD 2000 Series Graphics reached peak performance optimization on October, 2013. No additional driver releases are planned for these products.