The IIgs motherboard has some address decoding circuitry on it that is
used to set the state of two pins on the memory expansion slot that
tell the memory card what 1 MB bank of RAM to access. Because there
are only two pins (or bits) you can only have 4 banks represented that
way so only 4 MB can be addressed using these pins as stated in
various places.
However, there is nothing stopping memory card makers from putting
their own address decoding circuitry on their memory cards and being
able to access more than 4 MB. They might even be doing some other
tricks I suppose. There are a number of 8 MB memory expansion cards
in existance.
You could actually have a 14 (or even a 16) MB memory expansion card
but unfortunately the IIgs ROM has a bug in it which doesn't allow
access to anything beyond 8 MB. This is why you still only get 8 MB
in a ROM 3 IIgs when you install an 8 MB card instead of the 9 MB
you'd expect (8 MB on the expansion card and 1 MB on the motherboard).
KEGS and Bernie ][ The Rescue can allow a 14 MB system as the ROM is
not in hardware under an emulator and I believe it is a one byte patch
that fixes the bug. The fix is an option on these emulators because
no one really nows if it will mess up the operation of any software.
It seemed to work fine for me when I tried it.