resolution seems quite low on 4K screen

I just bought a 4K screen , but i found the view is quite blurry when using UE4 editor , it seems just like 1080p resolution , I tried set the resolution to 3840 * 2160 , but mothing changes , i don`t know what to do to make the image looks sharper ,right now ,the image is very blurry , event the icon looks not very clear , anyone encountered this problem before ? really appreciate it if anyone can help me !