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The Dell Venue 8 Pro 5855 is the first pure tablet with USB Type-C that we've seen that supports 4K@30Hz resolutions directly over USB Type-C to the LG 27 inch Class Ultra HD IPS 4K 27UD88-W Monitor.  The Google Pixel C Android tablet also supported USB Type-C, but upon initial release there was no support for DisplayPort.

To connect the Dell Venue 8 Pro 5855 to the LG 27 inch Class Ultra HD IPS 4K 27UD88-W Monitor, you need to use a compliant USB Type-C to Type-C cable like this one from Belkin.

On the LG 27 inch Class Ultra HD IPS 4K 27UD88-W Monitor, to find the USB Type-C port look for the one which has a DP DisplayPort logo and a battery-like USB SuperSpeed logo on top of it.  The other blue colored rectangle shaped ports are USB Type-A ports. Connect one end of the Type C to Type C cable to the port on the monitor, and the other end to the Type C port on the tablet.

After connecting the Venue tablet and LG monitor, you'll need to make sure you select the correct input.  Go to the LG monitor's Settings and select Input. 

Select USB-C to display from the USB Type-C input port.

Once the correct input is selected, you'll see the Venue tablet's screen being displayed on the the LG display up to 4K@30Hz.

You can see from the Venue's Windows taskbar battery icon that the Venue is also being charged while connected over USB Type-C to the LG display.

A note of caution: a USB drive connected to a USB port on the display will not be accessible to the Venue tablet.  USB devices are accessible through the LG display using other USB Type-C computers like the Google Chromebook Pixel (2015) and Apple MacBook (Early 2015).

The Dell Venue 8 Pro 5855 works great over with 4K@30Hz resolution over USB Type-C with the LG 27 inch Class Ultra HD IPS 4K 27UD88-W Monitor, but unfortunately USB devices connected to the LG display's USB Type-A ports aren't accessible by the Venue.