[FoxForceFive]

Are you using Kodi 18 RC5?

When you say it's not playing 4k files in 4k resolution how do you mean?

If I set it to auto switch framerate it auto switches my TV to 4k resolution from the 1080p I have Kodi set to.

Kodi switches to 1080p when I open it as I have it set to that in the Kodi settings.

Let me guess, you have gui resolution limit set to unlimited?

If so, when you play a 1080p or 720p file, check your TV resolution. In my case it is outputting in 4k, so the firestick is upscaling, which isn't ideal, cos TV's are almost always better at it.

I've opened a bug report, they've replicated it, worked out the problem & have even found a fix, so hopefully that fix will be in place in the nightlies soon.