Theoretically, Bluetooth should be backwards compatible. Accordingly, the Charge 5 should work at least as well as the Charge 4. Nevertheless, we cannot exclude that there might be a configuration/version problem. I have not found anything on the Github page of Bluealsa – the service we use for Bluetooth. But maybe you can find a hint that will help us: https://github.com/Arkq/bluez-alsa
Can you try how far away you have to move the Charge 4 from the Pi before the connection quality drops? It might be possible to solve the problem with an external Bluetooth stick, as the Pi Zero uses one chip for Bluetooth and Wifi and they can get in each other’s way sometimes.