If you're logging and reporting on ERRORs for 400s, then your error triage log is going to be full of things like a user entering a password with insufficient complexity or trying to sign up with an email address that already exists in your system.

Some of these things can be ameliorated with well-behaved UI code, but a lot cannot, and if your primary product is the API, then you're just going to have scads of ERRORs to triage where there's literally nothing you can do.

I'd argue that anything that starts with a 4 is an INFO, and if you really wanted to be through, you could set up an alert on the frequency of these errors to help you identify if there's a broad problem.