Dell WD19DC - 4K Not Working

@JohnIT  Quick additional note.  According to the Setup and Specifications document, the Latitude 7400's built-in HDMI port is only 1.4a, not 2.0, which means you'd only be able to run 4K at 30 Hz through that output, not the standard 60 Hz.  If you're satisfied with that (most people don't consider 30 Hz acceptable), then you should be able to run 4K 30 Hz even through a WD19DC operating in single connector mode -- as long as that's the only display connected to the dock.  But the fact that it's being forced down to 1080p suggests to me that you've got a second display attached to the dock.

@JohnIT  First, unless you also need to use that dock with a Precision 7000 Series system, the WD19DC is a bad dock choice for a Latitude 7400.  The DC variant is specifically designed for the 7000 Series systems in order to supply up to 210W of power by connecting over two USB-C ports.  Video bandwidth is also distributed across both connectors.  The DC is capable of operating in single connector mode, when it does that, it operates equivalently to the much less expensive WD19, NOT the Thunderbolt-capable WD19TB or even the DC when running in dual connector mode.  The only way to get 4K 60 Hz out of a WD19 -- and therefore out of a WD19DC operating in single connector mode -- is to have a source system that supports DisplayPort 1.4/HBR3 over USB-C.  But the Latitude 7400's Intel GPU only supports DisplayPort 1.2/HBR2.  With that type of system, you're limited to dual displays up to 1920x1200 each or a single display up to 2560x1600 each.  If you were to get a WD19TB, then assuming your Latitude 7400 has Thunderbolt 3 (which may have been optional on that model, can't remember offhand), you'd be able to run dual displays up to 4K 60 Hz each or triple displays up to QHD each -- although there are some restrictions on the combinations of ports you can use to get those higher-end setups.  Those are spelled out in the WD19TB's manual.  Make sure you look at the correct table for a Thunderbolt-capable DisplayPort HBR2 system.

If your Latitude 7400 doesn't have Thunderbolt 3, then you can't run a 4K 60 Hz display through any docking station that also sets up the USB-C link to carry USB 3.x data simultaneously.  You just don't have the bandwidth.