I bought Belkin High HDMI Cable, high speed with Gold connectors 2m - HDMI0018G-2M and
Startech.com Displayport To Hdmi 4k Audio / Video Converter Dp 1.2 To Hdmi Active Adapter - 4k @ 30 Hz
to match the 1.2 DP from the HD 4600.
The only thing I have noticed is that the leads for the Monitors are DVI-D single but ports are DVI -D Dual. with 1 x DVI-I single on the HD4600
However have also tried a heavier unmarked SKY TV HDMI cable which make no difference.