The short answer is "maybe".  Read on.

  Thanks to Michael Portuesi <portuesi@tweezers.esd.sgi.com> for this
  helpful summary:

  The 4D/2* has 16 memory slots.  You get access to them by removeing
  the right plastic cover and the metal shield underneath (box seen
  from the front).  The slots are in the upper, left corner (box now
  seen from the right).

  The slots have to be populated by SIMMs (some kind of industry
  standard).  I think 80 or even 100ns is allright, but take a look at
  the speed of your own SIMMs.

  SIMMs should always be mounted in groups of four.  In a plain 8MB
  4D/20 you have eight 1MB SIMMs.  They are placed in slots A and B in
  this figure:

      ABCD ABCD
      ABCD ABCD

  If you upgrade to 16MB using eight more 1MB SIMMs you simply insert
  the new SIMMs in slots C and D.  If you are going to mix different
  SIMMs you should always have the the same type of SIMM in slots with
  the same letter.

  As far as I know, the SGI 32MB memory upgrade is sixteen 2MB SIMMs,
  and they are mounted in all the slots.  Now, I have been told (but
  haven't tried it) that it is possible to mix 1 and 2MB SIMMs.  The
  important point is that the 2MB SIMMs should be in the lowest
  numbered slots.  To get 24MB you should populate the slots as shown
  (signatures are, 1 = 1MB SIMM, 2 = 2MB SIMM, 4 = 4MB SIMM, . = empty
  slot):

      2211 2211
      2211 2211

  The good news is that you can get 4MB SIMMs from third-party vendors
  outpricing the 2MB SIMMs available from SGI.  To get 32MB you mount 8
  4MB SIMMs like this:

      44.. 44..
      44.. 44..

  The bad news is that you cannot mix 4MB SIMMs with 1 or 2MB SIMMs
  (leaving a lot of spare SIMMs) and even worse, not all 4MB SIMMs will
  function properly.

  Among the "good" SIMMs are those from Toshiba.  They should look
  something like this (information I got from a news article posted by
  Chris Miller <eagle!news@ucbvax.berkeley.edu>):

            module ID tags:                 chip numbers:

     --------------          ---------      TOSHIBA
    | TOSHIBA      |        | 9025AAA |     TC514100J-80
    | THM94000S-80 |        | JAPAN   |     JAPAN 9020HDK
     --------------          ---------

  Among the "bad" SIMMs are those from Hitachi:

                                            chip numbers:

                                            JAPAN R200
                                            9026 2NN
                                            HM514100JP8H

  Other memory configurations that we have tried are (0 = empty slot, 1
  = 1MB SIMM, 2 = 2MB SIMM, H = 4MB Hitachi SIMM, T = 4MB Toshiba
  SIMM):

  	1100	1100		Came up as 8MB (correct)
  	1100	1100

  	1111	1111		Came up as 16MB (correct)
  	1111	1111

  	TT11	TT11		Came up as 64MB (wrong)
  	TT11	TT11

  	T000	T000		Came up as 16MB (correct)
  	T000	T000

  	TT00	TT00		Came up as 32MB (correct)
  	TT00	TT00

  	HH00	HH00		Came up as  0MB (wrong!!)
  	HH00	HH00

  	TH00	TH00		Came up as 32MB (correct)
  	TH00	TH00

  	TTH0	TTH0		Came up as 48MB (correct)
  	TTH0	TTH0

  	TTHH	TTHH		Came up as 64MB (correct)
  	TTHH	TTHH

  	11TT	11TT		Comes up as 16MB
  	11TT	11TT

  It appears as though the machine checks the first bank of chips (port
  0) to determine the chip size and assumes that the rest are the same.
  The Hitachi 4MB SIMMs are NOT correctly detected.

  It is important that the 4MB SIMMs in slot A are 'good'. Then you are
  free to use "bad" 4MB SIMMs in the rest of the slots (this is my
  experience), and it is possible to upgrade to 64 MB populating all the
  slots with 4MB SIMMs.

  When you do the actual seating of the SIMMs you should take
  precautions (wear a static strap, work on a static pad) not to damage
  the memory.  Sometimes you will have to reseat a module.  If a SIMM is
  not properly seated it will probably show up on the diagnostics
  terminal (if you have one attached) during power on.

  After a successful power on you should enter the PROM monitor and
  issue the 'hinv' command.  This should tell you how much memory you
  have (or how much the 4D/2* believes it has).  If this is correct you
  are ready to boot.

  Dave Olson <olson@sgi.com> adds: [The "good" vs. "bad" SIMM business]
  is a PROM bug.  We had a fix, but that PROM never released.  We didn't
  have 4 MB SIMMs when the last shipped prom released.  We read a memory
  location with the 4M SIMMs before we initialzed it.  SIMMs that power
  up all 1's work; those that power up all 0's require a reset or two
  (by then the memory was initialized).  The Toshiba simms worked once;
  I've heard that current 4M Toshiba simms may not.

  The moral of the story: many people do fine with 4M SIMMs in their
  4D25s, but don't buy them without a money-back guarantee.