I sincerely think OS X is the key element that prevents 4k @ 60hz over HDMI 1.4b.
I was one those people who bought an early 4k TV. It's best HDMI input is a 1.4b. Wasn't expecting much from this panel as I got it cheap, but it actually has incredibly low latency- great to game on. Windows 10 has no problem pushing 4k @ 60hz with HDMI 1.4b. It's flawless and works beautifully, but OS X (latest El Capitan) running on the same machine will not go above 30hz @ 4k. It's a software thing and it pisses me off. I've tried every type of adapter and display resolution app to force OS X to display 4k @ 60hz via HDMI and I've even tried fooling OS X into thinking it was outputting as display port with no luck. It's such a shame, because I know OS X is beautiful at 4k 60hz.
There are workarounds to get OS X to display at 4k 60hz over HDMI, but it requires an ''active display port to HDMI 2.0 adapter,'' a piece of software called ''SwitchResX,'' and 4k TV with HDMI 2.0.
I'm sure Apple has reason to prevent 4k @ 60hz via HDMI less than v2 standard (maybe unstability in certain instances), but man, it's annoying that Windows 10 can take advantage of this on the same hardware.
If I've learned anything from this experience, it's been that if you plan on buying a 4k TV, you better make sure it has at least one HDMI 2.0 port.