The LBP-4 memory expansion board is inside the printer.
To find it you must:

First, loosen the pair of screws that hold the plastic back panel
on; it is hinged at the bottom and will swing down:



That reveals a metal panel, with electrical connectors on it and
a big plastic handle in the top middle, and a pair of screws in
the lower left and right corners.  Remove the pair of screws,
grasp the handle, and yank:



The metal panel will come off:



That reveals the memory expansion slot.  It is in a "bay" on
the right side, and has a pair of plastic card guides (probably
light blue).



Kingston used to make these boards; their installation
instructions are here:
http://www.kingston.com/support/install/memory/Canon/KCNlpb4a.PDF


Possible part numbers for these memory boards are:

   s63-2230
   s63-2230+2240
   kcn-lbp4/1
   kcn-lbp4/2
   HCALBP4-1
   HCALBP4-2


Another way to tell whether the printer has the memory expansion
board or not, without opening it up, is simply to print a web
page full of graphics, or a full-page photo.  If it prints
the whole thing on one sheet, then it has one of the memory
expansion boards in it.  If it takes 2 or 3 pages, and prints
the picture in "stripes" (partial pages), then it has only
the base 512K of memory, like mine.


Regards,
-Dave