I have found the source of the problem:
The driver file Edward indicated had in its igdlhc64.inf a DevID of 041A, whereas my System lists Ven 8086 and Dev 040A. I modified the inf file to reflect the correct Dev code and presto, the driver installed. During installation it correctly identified the P4600 graphics HW.
Encouraged by this, I located the latest graphics driver at Download Center and made the same modification. This driver installed without a problem as well (except for the message that it is not signed).
So this problem is solved. Thanks, Edward for pointing me in the right direction.
Still unsolved is the problem that the driver does not support audio passthrough on the displayport.https://communities.intel.com/message/225196#225196