I'm not quite sure myself why all the 15.28 series drivers on the HD 4000 seem to get slower and slower with each release!
I'm still using the 15.31 2885 Beta drivers which have by far the best gaming performance on the HD 4000, I just wish they'd hurry up and release an official version of these as they also provide OpenCL 1.2 support too (which I'm currently programming for at the moment)
Could you please provide the full versions that you have tested with? Have you tried using the latest video driver and BIOS provided by your computer manufacturer? I believe that the latest driver for your system would be posted here: