What is the difference between HA docker images for Pi3 and Pi4

I am curious to to know the difference between Home Assitant docker images for Pi3 and Pi4, since the instructions mention two separate images when downloading.

I just want to keep around a spare Pi3/Pi4 for hardware failures, (also spare micro SDs), and I am wondering if the micro SDs will be plug compatible between Pi3 and Pi4.

For most of the other software I have installed on the Pi, it seems to make no distinction between Pi3 and Pi4, at least at the command line download and install. I might be wrong deeper down.