What OS are you using. If your using just Raspberry Pi OS then put the Pi 3 SDcard in to the Pi4 and turn it on. Your done. If using some other OS try just booting on the Pi 4 if it doesn't boot then you will have to start from the beginning.
Burn a new SDCard with a Pi 4 compatible OS. From there follow the instructions in tread "installJRMC - MC installer for Linux". Get a USB SDcard reader and insert into the Pi 4 with the old PI 3 SDcard. Copy the Pi 3's .jriver directory into the new Pi 4 .jriver directory. This sets up the catalog as found on the Pi 3. If you have music on the old card then move that to the new card. Try to keep the music directory location and name the same so the library links will work.
Run JRiver, register it and finish setting up. If JRiver can't find the music files then you may have to regenerate the library.
That's pretty much how I have been doing it across Pi 3 to Pi 5. If your using a hat card with the new OS then make sure the boot/config.txt file points to the cards driver.
