DeletedUser130 Pixel 4a (5G) and Pixel 5 have the 2023-11-05 patch level officially and were able to obtain the 2023-12-01 patch level via our extended support but not 2023-12-05 or beyond it. The YYYY-MM-01 and YYYY-MM-05 patch levels are split so that if you have the previous month's full patch level you can provide the YYYY-MM-01 patch level with only AOSP patches as long as nothing needing an AOSP patch is built into parts of the code you can't update such as a fix changing the API defined in C/C++ headers including changing inline functions/methods or templates which would require rebuilding code using that functionality in the header. We checked the 2023-12-01 patches and confirmed we had provided all of them.
They stopped getting firmware and MOST driver updates after their end-of-life. They still get partial driver updates via the Pixel 5a being supported. Pixel 5a has nearly the same hardware and it's very unfortunate the Pixel 4a (5G) and Pixel 5 didn't get their support extended until Pixel 5a end-of-life, but that's not within our control. It would have been possible to provide support for them for a bit longer. It might cost more money to license it from Qualcomm and it might have had to be paid for earlier. Regardless, they didn't get it so they shouldn't be used anymore.
Could the firmware of the Pixel 5a not perhaps be installed on the Pixel 4a (5G)
The firmware is different between each of the devices. There are minor hardware differences between them and since the Pixel 5a shipped later it's a later revision of the hardware with some improvements that were in progress for future devices.