Wotta lotta pixels
So what could you actually display on a 5K monitor that wouldn't be as good on a 4K one?
Well, the obvious answer for the average user is images from a 5K camera (and possibly the only answer: since video refreshes at rates that make seeing each pixel impractical - not to mention impossible, and once you can read a piece of text at a reasonably sharp definition, adding more hi-def. doesn't make it any better or easier to read - otherwise nobody would be able to use "old" 27-inch 1920x1080 screens to do that).
So still images it is. But wait! Even if you take an image from your DSLR, hasn't it been de-bayered inside the camera (and squished around to turn it into JPEG), so it's not exactly WYSIWYG any more. Going further: if you choose to take a squint at the RAW format, the camera still has an anti-aliasing filter in front of the sensor to reduce all those nasty Moire patterns. So you aren't even seeing the real image then, either - TIFF, JPEG or not.