You can monitor for the second case and react accordingly, you just don't monitor for them in the same fashion as 5xx class errors (which yes, despite what this

You can monitor for the second case and react accordingly, you just don't monitor for them in the same fashion as 5xx class errors (which yes, despite what this article is about, I'm still using that term). For 5xx errors you should typically monitor against a baseline of 0. 4xx class errors you want to monitor for substantial deviation from previous averages. This is a good indication if you broke something, vs client behaviour changed. Remember, monitoring isn't always going to "give you the answer", as much as alert you to a possible issue so you can investigate further. Moreso, in a SaaS application, monitoring for deviations in 4xx rate by client is also helpful. An alert on a single client is likely not a system-level issue, but a devaition across all clients likely is.