I used this set of instructions and the tool referenced in the first
comment myself:
https://www.youtube.com/watch?v=EJA4OUIDa7Q
to upgrade the firmware in a macmini 1,1 to 2,1, to allow it to see the
rest of the installed RAM. It was, for me, very quick and very painless.
As you noted, many macOS systems of this vintage (and other makes using
the same Intel chipsets) can only use a little over 3GB of the 4GB of
RAM due to hardware addressing constraints, even when running other
operating systems like linux.
Best,
Ken
