iMac 27 Late 2013 4K@60Hz Support

Hi all,

Really excited after reading what Apple has improved with 4K support (Last updated July 7, 2015). Therefore, I recently purchased an Asus PA328Q 4K monitor to be used with my iMac 27 (late 2013) as secondary monitor.

Using 4K displays and Ultra HD TVs with your Mac - Apple Support

The connection is through DP 1.2 cable, and the monitor has built-in SST over HDMI2.0 and DP/mDP ports. The setup is plug&play, however on the monitor it shows the resolution 3860x2140 @30Hz. I tried cycling through the monitor's option DP1.1/DP1.2, but with the same results.

In the Display preference page, I don't see any options to manually adjust the fresh rate to 60Hz, as in Windows 7/8/8.1. Does any one has successful experience with 4K@60Hz with monitors other than those listed on that support page? It does mention my iMac has SST and 60Hz support for "most 4K displays". Does this mean my monitor is not supported by the current Yosemite version yet, and Apple might extend the supported model list in later OSX updates, or with the future El Captain?

Any comment or suggestion is greatly appreciated.