Short answer: GA4 and Firebase measure installs with different rules from an MMP. GA4 uses a 30-day default lookback for first_open, Firebase counts reinstalls as installs, and GA4 app conversions only credit Google paid channels. Align definitions before comparing.
Why GA4 and an MMP count differently
Lookback window: how far back from a conversion a tool searches for an ad interaction to credit.
GA4 defaults to 30 days for acquisition key events such as first_open, with a 7-day option, and 90 days for other key events, with 30 or 60-day options. Your MMP's windows are set per network and may differ.
Can GA4 credit Meta or TikTok for app installs?
Not as paid channels. Google's help centre says app conversions in GA4 always use Google paid channels for the channels that can receive credit, so non-Google paid installs appear differently than in an MMP.
Likely causes
- Different lookback windows. 30 days for first_open by default in GA4.
- Reinstalls. Google says Firebase counts reinstalls as installs by default, while third parties often use a re-attribution window. See reinstalls inflating installs.
- Google-only paid crediting for app conversions in GA4.
- Session definitions. Firebase's default session timeout is 30 minutes and session definitions differ from third parties.
- Model changes apply to history. Changing GA4's reporting attribution model applies to past and future data, so old reports can change.
- Retired models. GA4 removed first click, linear, time decay and position-based models in November 2023.
How to confirm it
- Note GA4's lookback window for acquisition key events.
- Compare it with your MMP's windows for the networks involved.
- Compare Firebase first opens with MMP installs plus reinstalls.
- Split the comparison into Google paid, other paid and organic.
- Check whether anyone changed the GA4 attribution model recently.
How to fix it
- Compare totals first, then Google paid only, since that is where GA4 and the MMP share a scope.
- Add MMP reinstalls when comparing with Firebase installs.
- Use the MMP for non-Google paid channels.
- Record any GA4 model changes so historical shifts are explained.
- Compare counts, not values, as Google advises.
| Setting | GA4 or Firebase | Typical MMP |
|---|---|---|
| first_open lookback | 30 days default (7 option) | Per network |
| Other key events | 90 days default (30, 60 options) | Per network |
| Reinstalls | Counted by default | After re-attribution window |
| Paid credit for app conversions | Google paid channels | Every connected network |
The Google installs troubleshooter covers Google Ads gaps, the attribution windows guide compares windows across platforms, and which number to trust explains when to use each tool.