I'm having exactly the same problem - Intel HD 4600 on Dell U2713HM via HDMI doesn't offer resolutions higher than 1080p.
My mainboard is a Gigabyte H87N-WIFI, which clearly specifies 4K resolution via HDMI (but not DVI). The display features HDMI 1.4, which should enable higher resolutions.Yet I'm limited to 1080p.
Where is the custom resolution option in the Windows 8 driver panel? I saw it mentioned several times on other forums, but it doesn't seem to be there on the current driver version.