There are two reasons you may receive a 404 Page Not Found error after submitting a contact form:
Reserved Terms Used as Input
WordPress has a number of reserved terms that can cause issues if used outside of their intended context. If you use reserved terms for input fields in your contact forms, it can result in problems with form submission and result in a 404 error.
The solution is to alter your Form Tab field names so that they are unique to your form and don’t use any WordPress reserved terms.