At 4K I can set to 200%. Increasing above 200% on 4K would put some UI items outside of the screen, 300% would not be recommended.

I have a 1080p screen and run it at 4K when I am gathering screenshots.

I would have expected the UI scaling to work properly with true 4K when it works with fake 4K.

Head over to the troubleshooting guide, at the bottom are instructions for submitting a support ticket.

https://esupport.stardock.com/index.php?/default_import/Knowledgebase/Article/View/530/175/general-ashes-troubleshooting

The slider in the game options goes up to 200% for me, when I set it to 200%, the settings.ini shows 2.0.

If I set the UI scaling in the settings.ini above 2.0 then it will default back to 2.0.

If the UI scaling factor is calculating a maximum of 1.0 (100%) for your resolution then it will force it back to 1.0 if you try to modify the setting.ini file. It sounds like the UI scaling factor limiter is a bit too aggressive and not calculating the maximum scaling factor properly.