BluRays are 1920x1080 -- just as DVDs are 720x480. You can't make a 4k BluRay. Or, if you could, your BluRay player would not be able to play it.
In version 14 of Premiere Elements, use Export & Share/Disc/BluRay to create a BluRay disc of your movie.
That answer is "loud and clear".
Remains the question, why is 4K introduced for amateurs.
When my AVCHD camera (Sony) broke down, the only choice was a 4K camera.
OK, part of the progresion I thought, and bought a (Sony) camera, monitor and a new editing package to fulfill the new requirements.
After all, I could have saved a lot of money, altough photos on a 4K monitor are a real joy.
In fact I interprete you answer as: Put your camera back on AVCHD, use PREL 14 as in the past and make your BRD as usual.
I assume PREL 14 will not resist if 4K movies (MP4)are presented in the process of making a BRD.
Problem solved, user disappointed......
Don't forget, you still have the option of outputting your 4K video as a 4K mp4, which can be viewed on a computer or uploaded to YouTube.
So there is definitely a 4K workflow. There is just not a format for creating a 4K disc, like DVDs or BluRays.