I am looking to run 2 external displays, in addition to the internal display on my Lenovo Thinkpad T490s with an Intel UHD 620. I have access to the Thinkpad Pro Dock, as well as the thunderbolt 3 docking station. I currently have 2 external displays liked to the Pro Dock, however, I am being limited to a 1920 x 1080 resolution on one of my displays, although the display itself is capable of 4k. I am running both external displays to the dock via the two DP ports. 

Hello guest28318,

Welcome to Lenovo Community

I understand that you are looking to output 2 external displays with your ThinkPad T490s.
Based on the T490s' PSREF data sheet, the monitor support are as follows:
Monitor Support
Supports up to 3 independent displays via native display and 2 external monitors; supports external monitors via HDMI®
(up to 4096x2160@24Hz), Thunderbolt™ (up to 4096x2304@60Hz)
Have you tried connecting the monitors directly to the laptop?
Please provide the docking station's complete part number (e.g., 40ANY00135US) on your reply.
Update us soon as convenient.

Regards, 
Lorenzo_Lenovo

Hi Lorenzo, thank you for your input. To provide insight, here is what I am using:

LG 27GL850 (2560 x 1440 resolution)

LG 27GP950 (3840 x 2160 resolution)

Thinkpad Pro Dock 40AH0135US

When I connect the monitors individually to the laptop directly to use a single extended display, I am able to get full resolution. 

Hey Lorenzo, so for some reason in my display settings, I am unable to go down to 30 hz. 

The possible reason for the other monitor is with the bandwidth distribution so 4k at 30 HZ which means other monitor needs to tone down else you may need to lower the resolution. You can try plugging straight to the notebook to try since the laptop should provide such specification.
The 40AN dock model may not be compatible to the laptop but you may want to check out other models from the OCM (Options Compatibility Matrix).

Regards, 
Lorenzo_Lenovo

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Regards, 
Lorenzo_Lenovo