Notebook screens are single-resolution only -- if you're seeing 1920X1080, whatever you thought you'd bought isn't what you received. Go into setup at powerup (F2 a few times after the power button).  The native resolution of the screen will show in setup. If the screen was represented to you as 4K, then contact your seller/repair shop for an explanation.  
HI! My previous xps 7390 2in1 FHD touch screen suffered an impact and broke (i have a hyperactive 5 year daughter!)  I installed a used UHD replacement on the laptop.  But im unable to upgrade the native screen resolution to more than 1920x1080, and i would like to use the 4k resolution from the display. thks for your help    
Upgraded to a UHD native display but unable to configure more than 1920x1080- 
no 4k option