Depends if Android 14 releases before end of life or not, which seems very unlikely. Devices get support for newer Android versions directly from Google, Graphene just pulls those changes to update their OS to the newer version for supported devices. There will not be extra work by GrapheneOS developers to backport newer versions of Android to devices which never officially supported them. Meaning the 4a will not get Android 14 unless Google updates it to Android 14, which seems unlikely given that end of life is this month and Android 14 is probably not coming out until Fall at the least.
It will continue to receive partial security updates until the release of Android 14, anything after that is not guaranteed: https://grapheneos.org/faq#device-lifetime
Example: Pixel 3 reached end of life in November 2021, received partial security updates until August 2022, when Android 13 released, and GrapheneOS extended support therefore ended.