I have had 3 monitors connected for years and finally connected a fourth. Windows (8) will sort of detect it but not completely.
In where I change appearance of displays 1,2,3 are detected. 4 has a little window saying "display not detected", but it is in fact detected or that little window saying "display not detected" would not be there.
2 Monitors (1 and 2) are off the motherboard, 3 and 4 are off a pci card.
The one that does not work is connected to the DVI port on the card.
I have re installed the pci video card software again, but makes no difference
Suggestions please