So, here is my (temporary) solution, as reported in Bugzilla bug https://bugzilla.redhat.com/show_bug.cgi?id=2011423
So my 4GB rpi4 refused to boot with the symptoms described about, i.e. mmc0 errors.
I also found that Fedora-Minimal-34-1.2.aarch64.raw.xz and Fedora-Minimal-35-1.2.aarch64.raw.xz also failed to boot. So, I downloaded the latest PIOS image, cut an SD card, and (hey presto) it booted (so no problems with my pi, psu or other hardware).
However, there is a hint in the Fedora aarch64 wiki that one needs to remove /boot/dtb to get Fedora to boot (and also in the Bugzilla bug itself)). The only problem was that my Fedoras 34 and 35 both fell over without ever creating /boot :( So, here is what I did from my root history on my x86_64 Fedora desktop box:-
1002 # Put an SD card in USB card writer/reader. 1003 arm-image-installer --image=Fedora-Minimal-35-1.2.aarch64.raw.xz --target=rpi4 --media=/dev/sdX # Your X will vary. Use journalctl -f
1005 mkdir /mnt/usb 1006 mount /dev/sdX2 /mnt/usb/
1007 # You have remove the dtb soft link and the directory it point to:- 1008 rm /mnt/usb/dtb 1009 rm /mnt/usb/dtb-5.14.10-300.fc35.aarch64/ 1010 rm -rf /mnt/usb/dtb-5.14.10-300.fc35.aarch64/
1012 umount /mnt/usb 1017 gparted /dev/sdX& # Usegparted to enlarge /dev/sdf3 partition so that the whole image is < 10MB. 1018 parted /dev/sdX
1019 # Take a copy of the SD Card image and check that all partitions are actually there:- 1020 dd if=/dev/sdX of=Fedora-Minimal-35-1.2.aarch64-rm_dtb-size-10000MB.img status=progress bs=1M count=10000 1021 1023 fdisk -l Fedora-Minimal-35-1.2.aarch64-rm_dtb-size-10000MB.img 1024 # Put card into rpi4 and boot.
This works for F34 and F35 on my 4GB rpi4. I never had to do all this previously, but maybe these instructions will help some-one :)
Meanwhile, the Fedora wiki needs to be updated, and/or the Fedora .xz file images need to be fixed by some-one with the superpowers.
:D