FYI I tried the setup with an HDMI cable and the the 2560 x 1440 resolution works.
It could be related to the cable. We suggest a high quality straight HDMI cable.
Is the monitor model properly detected in Device Manager?
Yeah, we tried HDMI and then it worked. However, this monitor had previously worked fine with a DVI cable and an Invidia GT9500 card, so was surprised it didn't work with the HD Graphics 4600. We also allocated 512mb of RAM.