Hello,
Ive had this problem for a while and since i've found lots of internet support forums for the rpi4 which determined that the eth0 interface in the device was faulty. i gave no second thought to it and switched to wireless.
last weekend i've decided to look more into it and realized at some point in time i enabled systemd-network for the eth0 interface and also the netctl ifplugd service for the same interface.
This made the connection bounce on and off every 5 seconds making my life living hell.
naturally i disabled one of the services and the system was up and running again.
Leaving this here since it might happen to someone else too.
Cheers
Tomas