add troubleshooting section to GA4 (#2310)

* add troubleshooting section to GA4

* Update index.md

Co-authored-by: pwseg <86626706+pwseg@users.noreply.github.com>

Author: kdaswani

src/connections/destinations/catalog/actions-google-analytics-4/index.md

@@ -40,5 +40,20 @@ To add the Google Analytics 4 destination:
 
 {% include components/actions-fields.html %}
 
+## Troubleshooting
 
+### Custom Event Naming
 
+Google Analytics 4 does not accept custom event names that include spaces. Segment replaces spaces in the Event Name in the Custom Event action with an underscore. As a result, you will see custom events snakecased in Google Analytics 4.
+
+Google Analytics 4 is also case sensitive. If you would like all event names lowercased, use the `Lowercase Event Name` setting in the Custom Event action. If this setting is disabled, Google will treat event names with different casing as distinct events. For more information, see [Google Analytics 4 Event name rules](https://support.google.com/analytics/answer/10085872?hl=en&ref_topic=9756175#event-name-rules&zippy=%2Cin-this-article.%2Cin-this-article){:target="_blank"}.
+
+### User Metrics & Sessions
+
+Segment sends a Google `clientId` in requests to the Measurement Protocol API, alongside other required API fields. However, user sessions are [not currently supported by the Measurement Protocol API](https://developers.google.com/analytics/devguides/collection/protocol/ga4/reference/limitations){:target="_blank"}, so user metrics might not appear on the Google Analytics 4 Realtime report or several other reports. 
+
+To validate your implementation, check users on the Events report and User Explorer. You can also confirm events are picked up in the Realtime report.
+
+### Debug Mode
+
+The Google Analytics 4 [debug mode](https://support.google.com/analytics/answer/7201382?hl=en){:target="_blank"} only works with a client-side implementation through gtag.js, Google Tag Manager, or Firebase. Because Segment’s Google Analytics 4 integration is server-side and uses the Measurement Protocol API, debug mode is not supported.