Spaces and reserved names or prefixes are not allowed. Reserved lists can change, so check the official naming rules when designing a new event. Page_View and page_view are different names.
Distinguish automatically collected, recommended, and custom events. When implementing a standard action, follow its official event name and parameter requirements; do not describe all recommended event names as prohibited reserved words.
A team may adopt lowercase English and underscores for consistency, but this is an internal convention, not a lowercase-only GA4 restriction. When changing names, account for the distinction between newly collected data and existing history.