Exhort14 Thought I read that 4a was no longer supported, but perhaps that's coming for A15?
The link you sent, as of today, has three support classes. The 4a is listed as "extended support from GrapheneOS via a legacy branch based on Android 13 with only the Android Open Source Project security backports, certain other security patches, and other minimal changes to keep them working".
The Releases page says "Pixel 4, Pixel 4 XL and Pixel 4a are end-of-life and shouldn't be used anymore due to lack of security patches for firmware and drivers".
So, as of today, use of the 4a is not recommended, because the legacy extended support does not address all plausible security threats. A 4a running GrapheneOS is likely more secure than a 4a running Google's stock OS, but is also plausibly vulnerable to both remote exploitation and in-person exploitation.
It is possible that the legacy extended A13 support might end soon, but that has not yet happened.