Realistically the partition size is going to be 4GB and you’re going to have whatever is left assigned to user data. You’ll never be able to write user data to that 4GB (without resizing and moving the partition at least), so for all intents and purposes 4GB is being used for the system, even if not actively in use. I believe that on a/b systems the partition actually is double the size, as it’s got the new update (or old update) next to the running system.
Honestly I don’t really have a problem with this. I’m my experience, my actual system will be 3.7GB, but could easily be taken up by an update. I’ve also had issues with smaller partitions not being able to flash modifications to them.
4GB specifically for a raspberry pi build, and I believe I had to resize my 2xl to flash gapps on a rom