If those pre-compiled binaries aren't based on GPL software, I don't think you'll have much luck getting their sources.

Android's source code is a maze of branches and tags, unfortunately. The change can have been made in the generic cross-device kernel tree, or it could've been a separate module, or it could've been a binary.

One thing you could consider is checking out the kernel for a project like LineageOS: https://github.com/LineageOS/android_device_xiaomi_sm6150-co... They usually pull+filter changes from upstream. For instance, these changes may be of interest: https://review.lineageos.org/c/LineageOS/android_device_xiao... https://review.lineageos.org/c/LineageOS/android_device_xiao... These kernels are used for a variety of devices with similar hardware, so not all changes have anything to do with the Pixel 4a, but it could prove useful in your search.

Image.lz4 is definitely a Linux kernel, and it's definitely changed: https://android.googlesource.com/device/google/sunfish-kerne...

Would changes to external closed-source binary files change the kernel image itself? There are kernel modules in that commit but it looks to me like they all come directly from here: https://developers.google.com/android/drivers

Google does provide detailed instructions for downloading all sources for the kernel, and I didn't see any changes since 2023 to any source files: https://source.android.com/docs/setup/

Thanks for the extra places to look. I'll check them out.