As per the Ars Trchnica review:
Using a USB Type-C to DisplayPort cable (one of Google's, actually, hooray for standards), we were able to drive a 3840×2160 display at 30Hz. This is likely some limitation of the GPU, since the USB Type-C port supports DisplayPort 1.2. We had to put our Dell P2415Q into MST mode to make it happen, though, and higher scaling modes werent available. Performance is poor. In short, both the 2015 Airs and Pro are better choices if youre looking to drive an external 4K display.
So I don't think you will get 4k at 60Hz from these MacBooks, sorry