Surface Book can, according to Microsoft, support two 4K (3840X2160) monitors at only 30Hz. I can not find a way to set the refresh rate to 30Hz. In advanced settings, I only have 59Hz or 60 Hz options. I am running Windows 10, using the Surface Dock with two Display port outputs. The monitors are Samsung U28590D. The best I can do is run one monitor at 4K, the other at 1920X1080 with HDMI to display port cable. Two monitors at 60Hz, 4K resolution will not work. Any ideas of how to set the refresh rate to 30Hz, would I want to do that anyway? Are there any external adapters that may do the Job?
That's weird that the Samsung U28590D doesn't offer 30 Hz with DisplayPort. One way around that is with an EDID override, but I don't think there's a way to do that with a Surface Book because it has an Intel GPU. The other way is to use Intel's control panel to add a custom resolution, but I don't think the driver for the Surface Book includes the Intel control panel. You can try downloading the official Intel driver (get the .zip version) and installing it manually through the Device Manager:
https://downloadcenter.intel.com/download/25951/
The other option is to use HDMI for both monitors. You should be able to get 3840x2160 @ 30 Hz with HDMI, but you need to use active adapters.