Setscissor (0,0,0,0) cuts out any black boarders, it does not stretch out the screen.
Setdisplayaspect(-1) works on all devices as it displays everything in percent's.
You have to make sure you position your images with in 100,100
What I do is load my images and set there size to 100,100
And to position them correctly to set there positions to 50,50
So if you use this, make sure you set all your images with in the 100,100 range,
As for the Pixel 4, I can not answer because i do not own one.
And yes, you can try SetImmersiveMode(1) as above.