4K monitor does not support 60HZ via HDMI on 3840x2160

I am confused about HDMI -- I have a GTX 770 with 1xDP, 1xHDMI and 2xDVI. I just bought two Dell Ultra HD 4k Monitors (P2715Q) and while the first one gives me 60HZ over DP, another only shows 30HZ over HDMI. Does it mean that I need to replace my video card with the card that has 2+ DP ports? Or it means that my HDMI cables are crappy? I tried 2 different HDMI cables...

The GTX 770 specifications page states:

3840x2160 at 30Hz or 4096x2160 at 24Hz supported over HDMI. 4096x2160 (including 3840x2160) at 60Hz supported over Displayport. Support for 4k tiled MST displays requires 326.19 driver or later.

So yes, you will need a graphics card with either two DisplayPort connectors or one that has HDMI 2.0 rather than the HDMI 1.4 that is supported by the 770.