Hi Michael,
From hardware point of view producer said that RPi 4 has full backward compatibility with RPi 3, but I didn’t check it by myself yet.
If you want to use full functionality of CORE2-ROS (cloud and ROS) you have to use our image or install our package to make SBC start cooperating with CORE2. But If you just want to publish and subscribe topics you can do it using USB cable and this tutorial. Types of messages between melodic and kinetic distro should not be different.
Best regards,
Hubert