Instructions are based on Ubuntu 16.04 but works just the same on newer versions: https://retropie.org.uk/docs/Debian/

The favorites are stores in the gamelist.xml file of each system. As @Lolonois said, if you copy the config you should be alright.

@sibsonian said in Migrating a build from RPi3 to Rpi4?:

I know the configs dir will migrate the artwork and ES settings. Common sense would lead me to believe that I would also need to reinstall any optional packages and themes, yes?

Correct.
You will need to install the optional packages that don't come standard on the image.

What version of RetroPie do you have currently installed ? If you used an image version older than 4.6, then you'll need to install from a new 4.6 image on the new system.

To transfer your existing files, backup your ROM/BIOS/Config files from the 3A+ installation, then restore them on the new installation.

@saccublenda as cunning as a rock that fella!

Just go to \\retropie\ and copy the roms folder to your desktop so you can can send it to any future pies.

ALl I had to do was move the NV files as well as all emulator named folders under roms\arcade over and all is great!