The Intel® HD Graphics 4600 supports the following refresh rates:
DisplayPort* 1.2 / eDP*
H-Processors: 3840 x 2160 @ 60 Hz (Ultra-HD)
U-Processors: 3200 x 2000 @ 60 Hz, 3840 x 2160 @ 30 Hz
Y-Processors: 2560 x 1600 @ 60 Hz
HDMI*: 4096 x 2304, 3840 x 2160 @ 24 Hz / 24bpp
DVI: 1920 x 1200 @ 60 Hz
This information is available here
I am running DisplayPort 1.2 at 2560 x 1440 @ 120hz on HD Graphics 4600 literally right now as I'm typing this on this 4720HQ (H) Processor.
According to the documentation, this is not supported.
Assuming the actual limit is 5Gbps (3840 x 2160 @ 60 Hz) then the equivalent bandwidth at 2560 x 1440 would be 135hz, yet anything about 120 gives me "not enough bandwidth" error. I don't really understand what's going on.
My best guess is that the refresh limit in the drivers are set at standard denominations (i.e 60fps, 100fps, 120fps), and since 144fps is above the bandwidth limitation of 5Gbps, anything between 120 and 144 can't be done despite the processor being capable of rendering it.
Although processor could be capable of rendering 144 fps and monitor supports these rates and resolution, running Display-Port 1.2 at 2560 x 1440 @ 120hz on HD Graphics 4600 is not a supported integration thus, errors are expected.