So here we go … i have an experimental Ubuntu Core18 image for the Pi4 up at:
Please see the README file in that dir for details …
The u-boot tree used in the gadget is still in very early stages (mainly a pi3 u-boot with the most minimal patches to make the board boot at all and a binary pi4 dtb stuffed on top) and does only initialize 1GB of RAM even on pi’s with more physical RAM available.
Links to the sources for the snaps as well as a subdir with the model assertion used to build the image can be found under the above URL as well.