I notice a trend here, in that every other post is about 4k not working at 60hz on the 620/630 chips
I'm in the same boat. Bought an Asus UX430 (8th gen, UHD630). It only has USB-C/TB/eDP output. Connected to my 4k display (LG 49UJ6300) but only lists 30hz and under.
For reference, my nearly 6 year old ROG laptop connected to the same display via hdmi has no issue getting 4k@60hz with 12ms input lag. I've tested with several other Nvidia GPU's and they all work as well.
So I figured maybe Asus somehow borked their setup, so I talked to Razer about the Blade 14 (2017) and they were absolutely positive that their USB-C/TB port would support 2x 4k displays at 60hz each, when connected via a USB-C/TB > HDMI2.0 dongle. So I bought one. Guess what. Also doesn't work.
I've tested both in Windows and Linux with the last 5 most recent drivers from Intel. Same results every time. The Blade has a discrete 1060 as well, so I connected it to the same TV. Works fine at 60hz.
There are two common denominators I've seen in every single post regarding the 620/630 and 4k @60hz, and that is the chip itself, and the use of the USB-C/TB/eDP > HDMI2.0. I know there are a ton of bad adapters out there, but mine has been tested and proven to work by multiple people. I did find a few threads from a few months back, which suggested several people had got 60hz working (through the USB-C/TB port) by "upgrading" to the then most recent Beta driver. That was driver I believe was Beta_18.104.22.168.4474. They ALL said that when they updated to the most recent stable drivers, their 4k dropped back down to 30hz.
Any idea's? I've only got a few days left to return these laptops. They're useless to me without 60hz. Can't even mouse properly at that speed.
*EDIT* Just added another test to the list. Realized my Asus UX430 also has a miniHDMI2.0 port so I tried that, again to the same display. Same result. 4k@30hz.
Also just for giggles, I tested all three setups successfully at 1440p@60hz and 1080p@120hz.
So to be clear, my HDMI cable isn't the issue as it works in 4k@60hz from any nvidia GPU dating back to the early 7xx series.