You may just need to edit your INIs to allow the texture streamer to stream in 8k. By default it limits to 4k just because using too large of textures for video cards with low memory can really cause the editor to hitch.
This post has more info
https://forums.unrealengine.com/showthread.php?70493-How-to-set-in-game-texture-resolution-to-8K&p=296198&viewfull=1#post296198