Gnome 3.26 doesn't scale up on External 4k monitor

HI people!!
I’ve a Dell Inspiron 5548 with 1300x700 (more or less) screen and when I try to connect a 4k external monitor, everithing apart the wallpaper is sooo small: windows, bars, webpages ecc…
Are there any settings to be modified?
After googling a bit it’s not clear whether is a bug or a feature not yet implemented or else.

Thanks, any help would be appreciated!