To make it easy to experiment with different OS images, I fitted a 15cm SDCard extender* from Pimoroni so that I can change SD cards without doing any dismantling. I used a bit of double sided tape to stick the card holder on to the edge of the case. You can just see the card protruding slightly to the left of the screen in the photo below. Out of the box, an RPi4 tries to boot from the SD card and if that fails it looks for a USB device. So, if an SD card is fitted, the Portable Pi boots from that an if none is present in boots from the Integral SSD. This does slow booing up a little but it is really convenient for experimenting.

* Some have reported problems with using this on an RPi4 but it worked fine for me - see Pimoroni website.

As you can see from the photo, I installed Ubuntu with no problems on the Portable Pi from the image available from the Raspberry Pi Imager utility. Ubuntu selects a 1024x576 (15:9) screen resolution. I only had to adjust the icon size and Terminal text font size to make it quite useable.

Overall, it is a bit slower to load applications than the native OS and I had the impression that it was a bit slower at rendering webpages on Chromium, so for the moment, I am sticking with Bullseye.