My primary relay runs on a Pi 3B. 903CA67D0DEB74CFBB01432840A26CB8C6C18FDF. According to the logs, it's sending and receiving a total of about 130 GB per day. A Pi 4 should definitely be able to handle it. Pi Zeroes had a conflict with the name of the CPU architecture and so Tor apparently never worked quite right on them.

I'm not an expert, but my guess is that the relay just had to reset its advertised bandwidth and has to build back up again. Hope this helps.