how to set 3840 pixels wide for external 4k monitor
Apple support officially a small number of 4K Display. https://support.apple.com/en-us/ht6008

So, If you want to display your 380x2160 resolution, you can try SwitchResX. That allow you to display your own custom resolution to the screen.
http://www.madrau.com/srx_download/download.html