I don't think there's anything Pi specific on the SD cards, so I think I can just pull the card from a 3B and stick it in a new Pi 4B and all will work.
Obvipously I'd do a backup first just in case.
Each new Raspberry Pi model usually requires some additional support in the Operating System (OS). The newer OS almost always supports older models.
What that means is that the version (release) of the OS installed on your RPi 3B will determine if it supports the RPi 4B. cat /etc/os-release to determine what release you are using.
RPi 4B requires "Buster (10)" or better. RPi 5 requires "Bookworm (12)" or better.
Just make sure to run sudo apt update && sudo apt full-upgrade -y on the pi3 before moving the SD card to the Pi4. That will ensure you have the latest system files on the SD card for the pi4
1 Like
Thanks, is it worth doing this on the existing 4B Pis as well?
Certainly worth doing a sudo apt update && sudo apt upgrade -y.
1 Like
That gets me to VERSION="11 (bullseye)" not the above mentioned bookworm
Bullseye (11) works on every Raspberry Pi model except the 5 so I believe that accomplishes your goal of upgrading your RPi 3B to an RPi 4B.
If you want to keep going, then see https://www.cyberciti.biz/faq/update-upgrade-debian-11-to-debian-12-bookworm/.