Chris Green wrote:

Surely the kernel can only 'see' 4Gb of memory on a 32-bit machine

PAE allows memory up to 64GB with a 32bit O/S

For Windows, some drivers were found to be buggy when dealing with more
than 4GB, so for desktop versions this was artificially limited to 4GB
(there are hacks to switch off the 4GB limit) and only allowed on
servers where better qualification of drivers was enforced.

so it can't hand more than a *total* of 4Gb to all the processes
it's managing. That is unless there's some clever memory management
hardware in addition to the 32-bit addressing.

An extra level of page tables.

Depending how they're tuned, server versions of windows allow either 2GB
or 3GB of memory per process, or there's an API that allows memory
hungry programs (e.g. SQL databases) to 'bank switch' memory to access
more than 4GB in a single process.

https://docs.microsoft.com/en-us/windows/win32/memory/address-windowing-extensions

All in all, for many years, it's been easier to use a 64bit O/S.