I have one 6900 series and a 4600 series video card installed in my system, 3 24" monitors are connected to to the 6900 two using HDMI and one using DVI, a 19" monitor is connected to the 4600 video card using HDMI, they are all setup as extended desktop and all worked fine until I ungrade from 12.6 to 12.8 or above Catalyst drivers.  Once newer drivers are installed Windows 7  (64 bit) disables the 4600 series video card in Device Manager and Catalyst does not recognize it as well.  If I un-install all drivers and go back to 12.6 all 4 monitors work fine again.

This is the latest driver for your 4xxx card > http://support.amd.com/us/gpudownload/windows/legacy/Pages/legacy-radeonaiw-vista64.aspx

also > http://forums.amd.com/game/messageview.cfm?catid=444&threadid=162109&enterthread=y

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