A:Answer 1. This drive is a 3.5” drive, not a notebook 2.5” spinning or solid state drive.
2. Many Mac’s only come with flash memory, so upgrading in a sense would just mean a build to order upgrade. 4TB would be very expensive for flash memory.
3. If you need exorbitant amounts of storage, the macs can be Booted from an external hard drive and that would give the option of very large hard drives. Much cheaper than Internal flash memory. My Samsung 2TB Ssd cost $ 120. 2tb flash memory in a Mac mini was $1000 extra.
Check with Apple for storage limits, but with 64 bit OS ie catalina, storage should not be an issue.