Have you checked the services of the WAS directly, if you see the 404s there? It should show them, even though they don't count towards the failure rate on server site.
Maybe the selected Management Zone or Timeframe is not correct?
It seems that Dynatrace doesn't consider 404 errors as failures by default.
But you can edit the service settings to consider 404 HTTP response as failures.
I provide you some steps to do this
1. Locate the service in dynatrace
2. Edit failure detection settings
Also if you would like to this but globally for the tenant, you can configurate it in the tenant settings
settings >> server-side monitoring >> failure detection
Tell us if this works