Hope for 4k 60Hz on low-end 15'' rMBP?
According to the below link which includes the release notes of the beta 10.9.3 13D57 build, 10.9.3 provides "improved 4K display support on .... MacBook Pro with 15-inch Retina display (Late 2013)".
http://9to5mac.com/2014/05/01/os-x-mavericks-10-9-3-release-nears-with-bolstered -4k-display-support/
As you would remember, the high-end 15'' late 2013 rMBP with Nvidia 750M can do 4k @ 60Hz on Mavericks by virtue of the 10.9.3. This release note gives some hope to the owners of low-end late 2013 15'' rMBP with Iris Pro because the release note seems to be written down in a way that it encompasses all late 2013 15'' rMBPs, i.e. both low-end and high-end models with integrated Iris Pro and discrete Nvidia 750M GPU. In short, low-end late 2013 15'' rMBP with Iris Pro may do 4k @ 60Hz once the 10.9.3 final version is released. At this point late 2013 13'' rMBP with Iris has not been provided with such support. In other words, no 4k @ 60Hz on Mavericks with late 2013 13'' rMBP for the time being. Hopefully some day, perhaps with the 10.9.4, it will do it. Because it has been proved that it can do it on Windows. Therefore it can do it on Mavericks as well with the necessary driver.