Anyway it should be able to shoot 4K for at least about 8 minutes! I usually can take about a 10 minutes video on 4K! And it's NOT normal operation of the camera! 4K recording requires a lot of power! So the phone heats up! Z2 is very good in that manner because even the GS5 which is made of plastic which should be able to disperse heat more efficiently than glass or metal, cannot record more than 5 minutes in 4K
Are you sure you have an updated device!? Because this issue was there in some old firmwares!! Check whether your build number is 23.0.1.A.0.167! That's the latest firmware and if you do not have installed it, update your phone and see! It should solve the problem!