Sony Xperia Z5 Premium only switches to 4K in certain situations

[cdmagurus.com]

4K mode on the Sony Xperia Z5 Premium is only enabled when viewing photos and video, but that's it. You won't get a constant stream of 4K goodness playing games or with general use of the OS.

More... (http://phandroid.com/2015/09/15/xperia-z5-4k-display/)