If you want to use a single image for responsive design then the monitor size really doesn't matter as much as knowing the maximum size of the content area of your website. Even if you do not want to use media queries for your images, you need to be using media queries for the breakpoints in your major site structural elements (like content area, banner, sidebars, ...). Therefore, if even if a visitor has a 4K monitor, your content area will likely be much smaller based on the maximum size media query breakpoint (if your content area can really grow to the width of a 4K monitor then you have design problems).
For example, even on my 5K iMac, the latest site I developed using Bootstrap framework has 9 columns for the content area and 3 for the sidebar (auto left and right margins keep the site centered in the browser window of course). This results in the content area having a maximum width of 836 px. Therefore, I have set Drupal 8 to resize all images uploaded by editors to a maximum of 836 px. Make that image responsive using CSS and you are good to go.
For another example, look at the current page in MacRumors in developer window of your browser as an example. If they wanted to allow posting of images to posts, there is no reason to allow larger than 777 px because that's the max content area no mater how big a monitor the visitor has.
