[gquiring]

Which picture screensaver would this work with? The one I'm using only has coordinates in the xml for 720p, so I'm assuming this wouldn't display images at 4k?

Picture Slideshow Screensaver. The XML values don't matter as Kodi will scale to whatever resolution it's running at.
 

There is no option for GUI at 4k, just unlimited and auto and neither do 4k.

 

• Resolution - 3840x2160p
  
• Set GUI resolution limit - Unlimited
  

You must be assuming it works this way, without actually testing it.  None of the screensavers run at 4k.  All of my photos are 2k and higher resolution and not one screen saver displays higher than 1080.