TL;DR USB-C Docks that include USB 3 data ports most likely do not support DisplayPort 4K at 60 Hz unless they support DisplayPort 1.4. A dock with a DisplayPort port and USB 2.1 probably can do 4K@60Hz.
I recently bought a BenQ 4K monitor and when I connected it to the USB-C dock that I had I found out that the dock could only support 4K at 30HZ, which is noticeably slow just when moving the mouse on the screen. Just about all the computer monitors we use support 60 Hz refresh rates.
Now, I was using my Pixelbook with the dock and I found that when I connect it to the monitor using a USB-C cable it can do 4K@60Hz, so I figured the dock was the limiting factor, which I confirmed on the manufacture’s web site. I then started to search for docks that say they can do 4K@60Hz and decided on the Anker PowerExpander.
As you probably imagine by now, I got the new dock yesterday and found it also is only doing 4K@30Hz, on the HDMI and DisplayPorts. Sigh. I tested the dock with another computer and it does 4K@60Hz, so I am back looking at the Pixelbook.
I have since learned from this article that the problem is that USB-C docks need to support data transfer as well as display and that decreases the number of lanes that are available for display at high refresh rates to two, whereas 4K@60Hz needs all four lanes. When you connect the Pixelbook to a monitor with a USB-C cable it can use all four of the cable’s communication lanes to enable it to handle 4K@60Hz.
Anker’s site says 4K on DisplayPort requires DisplayPort 1.4 and I have found that the Pixelbook only supports DisplayPort 1.2. As the article I found explains, DisplayPort 1.4 only needs two lanes for 4K@60Hz because it has an additional high bit rate mode and compression, which is why the Anker hub I have can do 4K@60Hz while also providing USB 3.1 data transfers. The problem is that the computer, hub, and monitor all need to support DisplayPort 1.4 for this all to work, and the Pixelbook does not.
The lesson from this experience is that connecting to high resolution monitors at high refresh rates via a hub is more complicated than simply using a cable because it requires understanding the technical capabilities of the computer, hub, and monitor. You may have to dig to find out whether a computer, and a Chromebook in particular, supports DisplayPort 1.4. Thunderbolt 3 also supports 4K@60Hz and fast data transfers but is not yet available in Chromebooks and Thunderbolt 3 docks are more expensive than USB-C docks.
I think my experience that I’ve written about here explains why Google recently announced the addition of docks to their “Works With Chromebook” program. If Google were to announced a new Pixelbook this summer that had DisplayPort 1.4 support (or Thunderbolt 3) it would be helpful to know exactly which Docks can do 4K@60Hz.
As for my current situation, even though this new Anker dock cannot provide 4K@60Hz with my Pixelbook, it has more display ports to drive multiple monitors and has more data ports than the hub it replaced, so I still have an upgrade over the dock I was using previously. However, had I known what I now know prior to purchasing this Anker dock, I probably would have bought this Cable Matters dock instead as I could live with USB 2.1. For now I using the slightly lower 2560 x 1600 resolution at 60Hz rather than 4K, but if I really want 4K I can connect directly to the monitor via one USB-C port and use the dock on the other for power and the additional ports.