Hello!
I’d like to use a non-full screen layout for my game. How can I achieve this?
Thanks!
In the upper corner of your 3D perspective viewport there is a small white button that will “minimize” the viewport into a 4-way view.
After that, it’s just a question about moving the different tabs around as you please until you have the setup you want.
Thanks, Sentura! But I meant the actual game, not the editor. 
Ah sorry, it was a bit unclear from your description.
What you could try is making a widget setup that has space for your GUI elements in the locations you want (make the containers fit), then have a separate widget with a camera feed for the viewport part of the UI. That way the viewport will not fill out the entire screen, like requested.
For the camera feed, you can use the same setup as for a minimap, as shown by this tutorial: Beginner Tutorial - How to make a minimap in Unreal Engine 4 - YouTube
The basic idea is to have a SceneCapture2D actor setup with a render to texture, then putting that into a material, and finally, putting the material into where you have your viewport widget.