Pssst. Pssst! Got a Sony Xperia XZ Premium?
If so, then you're probably awed by its super-crisp 4K display... that's actually downscaling content to Full HD 1080p by default.
Yes, as per Sony, "4K (and 4K HDR with 4K HDR content) is used in the Album and Video apps and third party apps with 4K (and 4K HDR) content. Everything else, so homescreen, apps, menus, etc are shown in Full HD."
A bummer, isn't it? Having a 4K display on paper but not using it to its full potential... Well, lucky for all of you tinkerers out there, there's a neat little trick that allows you to easily force the Xperia XZ Premium to output 4K all the time. Of, course, this is likely to have a detrimental effect on your phone's battery life and performance, especially in games and other graphics-intensive tasks.
A bummer, isn't it? Having a 4K display on paper but not using it to its full potential... Well, lucky for all of you tinkerers out there, there's a neat little trick that allows you to easily force the Xperia XZ Premium to output 4K all the time. Of, course, this is likely to have a detrimental effect on your phone's battery life and performance, especially in games and other graphics-intensive tasks.

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So, how do we know that the Xperia XZ Premium is not running at 4K all the time? First up, open up a system info app of your choice and check out the resolution. In our case, it's 1080x1920 pixels as per AIDA64, and we are fairly certain it will be the same in your case as well.
Step #1

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You will need to open up an ADB shell. In case you're not aware what ADB is, we're not certain you should be here, but anyhoo, there's a first time for everything. First of all, you need to download the ADB SDK from here, then make sure you have the latest version of the Google USB driver. Install both of these on a PC of your choice. Alternatively, you can go with the Minimal ADB And Fastboot Tool, which is a less of a hassle to install and get around.
Step #2

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Enable Developer Options on your phone if you haven't already, and make sure that USB Debugging is enabled. Launch the ADB Fastboot tool, then connect your phone to your PC. A pop-up message will appear on your phone, asking you to authorize the connected computer to debug it, which you should allow.
Now, in the ADB shell window, key in "adb devices" and hit Enter to make sure your Xperia XZ Premium has been successfully connected. If yes, proceed to the next step.
Now, in the ADB shell window, key in "adb devices" and hit Enter to make sure your Xperia XZ Premium has been successfully connected. If yes, proceed to the next step.
Step #3

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Type in "adb shell" then hit Enter, then type in "wm size 2160x3840" (no inverted commas) without the inverted commas and hit Enter again. Back on your phone, you should notice a flicker and a sudden change of the resolution as well as the scaling of the interface. To fix that, just type in "wm density 820" and hit Enter one final time. The interface should be better-scaled at this point. And that's how get your Xperia XZ Premium to output 4K all the time.
If you want to go back to the default resolution output of the phone, make sure to backtrack by inputting "wm size 1080x1920" followed by "wm density 420" in the ADB shell.
If you want to go back to the default resolution output of the phone, make sure to backtrack by inputting "wm size 1080x1920" followed by "wm density 420" in the ADB shell.
Step #4
Sony Xperia XZ Premium - here's how to enable 4K output all the time
So, how do we know that the Xperia XZ Premium is not running at 4K all the time? First up, open up a system info app of your choice and check out the resolution. In our case, it's 1080x1920 pixels as per AIDA64, and we are fairly certain it will be the same in your case as well.
reference: @damirfranc