The DisplayPort settings of the LCD display may need to be configured to achieve maximum resolution using a DisplayPort connection.
Your Dell 4k or 2k Ultra HD display may report that the maximum resolution offered for the display is less than 3840 x 2160. The highest resolution obtainable may show as WQXGA 2560 x 1600 pixels.
This can occur when the display is correctly connected using a DisplayPort cable without adapters.
To ensure display resolution higher than 2560 x 1600 pixels, configure DisplayPort input to DisplayPort 1.2 in the On-Screen Display (OSD) settings of the LCD display. For information on changing the OSD settings for you Dell LCD display, reference the display manual at the following location: Dell Support / Manuals.