how to set 3840 pixels wide for external 4k monitor On a Macbook Pro early 2013, I am trying to run a 4k Samsung TV.  It's almost working (though the text is awful, I think it's just that the TV is not really suitable).  The issue I want to ask about is that the Mac is sending a 4096 pixel wide signal to the TV, instead of 3840.  The TV accepts the signal fine, and then trims off a strip from the left and a strip from the right.  In other words, it's doing what used to be called overscanning. 

This leads to critical parts of the picture missing, because of the controls that live in the top-left and top-right corners of the screen.

The TV has a function where it tells you the input characteristics, and it indeed reports 4096x2160.

How can I tell the Mac to run the external display at 3840x2160 instead of 4096x2160?
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