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

You can search for them on eBay.  They used to come up for sale
there occasionally, maybe they still do.

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 did before I added a memory
expansion board.


Regards,
-Dave