Resolution Issues:
1. In order to enjoy the 4K high resolution, please make sure to use HDMI 2.0 cable.
2. Supports resolution up to 4k@30Hz,but we recommend you to make sure your monitor/tv could supply corresponding output. That means if your computer supports 4K@30Hz, while your monitor/TV only supports1080p, you can only output 1080p rather than 4K.
3. If the resolution of your laptop is higher than the external monitor.
For windows: Windows Menu--> Setting--> Display--> Advanced Display Settings, set your laptop's resolution same as or lower the resolution of external monitor.
For MacOS: Left-upper Apple icon-->System Preferences-->Displays-->Mirror displays. Then your HDMI monitor would mirror your Mac. However here you have two choices. One is "Optimize For" your built-in Retina Display, the other is "Optimize For" your HDMI monitor. You can choose which one you like.