1) Please check whether your external display support 4K resolution. If the laptop supports 4K but the display does not support 4K, it will not extend the 4K resolution of the laptop;
2) If you are using a Windows laptop, please go to
https://www.intel.com/content/www/us/en/products/docs/graphics/graphics-command-center.html to download Intel graphics command center.
Here, you can clearly see the default resolution of your laptop's graphics card. And it will automatically make a decision that best suits its output resolution.
3)If you want the external monitor to achieve a higher resolution, you can adjust it manually. I have attached a picture for you to guide you through the setup operation. But if you are here, you still cannot set a higher resolution, either your laptop does not support the external display to reach a higher resolution when multi-screen expansion or your external display does not support a higher resolution Resolution.