Tristin, that is an excellent question. And I have no answer for you right at the moment.
The 4K monitor thing kind of snuck up on me. I have a Retina screen MacBook Pro, but at maximum rez I'm only at 1920 x 1200. That is way less than your 4000 x 2000. Wow!!
Currently, the Simple Machines forum software limits the filesize of one attachment to 2560kB = 2.56mB. Total combined attachment filesizes in one post are limited to four times that for a maximum = 10.24mB.
The forum software limits the
page display size of a file to fit inside a 1000 x 1000 px box. If you right click the image, select View Image and then click the image again, you can see the image larger than 1000 x 1000 px. To see images as large as possible the browser must be fully expanded with no sidebars. I'm not sure how the maximum click-up View Image size is determined.
[EDIT: I had the page display maximum size incorrectly stated and so edited that.]