The Intel board can support upto 3 independent displays, with any rate supported as long as the display has supporting drivers for anything non standard going beyond PAL/NTSC 1080P.
You have to consider the GTXm in it too, because you may be able to set or tweak things on the nvidia control panel.
I haven't tried to experimented, but the mini dport might have issues running 4K screens for gaming. I dont have a 4k display panel. hdmi out is limited, like you said.
The worst that happens is you get capped at 144Hz, and keep the monitor for a more powerful notebook for the future.
I thought the TB controller on the G751 was Gen1? Bonus if it's 2... I just got a reply on the Intel boards when I was asking about cross compatibility and making a USB3.1 driver for TB1 on the G751.