I can't help you with this issue, but could you tell me how did you bring this up?
Yeah, I've got a 55X750F, and this is the exact reason why I ended up putting a Chromecast Ultra on it. I was stunned that Sony didn't enable 4k on the built-in Chromecast.
Why wouldn't the builtin Chromecast support 4K output? The TV is Sony Bravia Android TV 4K model.