Why can't my external monitors reach 4k resolution?

1) Please check whether your 3 external displays 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 the VGA port uses a 1080p display, in the duplicate mode, it will pull down other displays to 1080p;

3) 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.

4) 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.