IBM Model M4 & M4-1 Space Saver Keyboard

Also applies to Unicomp Mighty Mouse



Model M4-1 numeric keypad and keyboard
Model M4-1 numeric keypad and keyboard

The IBM Space Saver Keyboard and later Unicomp Mighty Mouse (codename "Surf") series were low-profile desktop peripherals originally introduced in 1992 and produced until 2010. These were the marketing names of the Models M4 and M4-1 buckling rubber sleeves keyboards, which in turn were desktop/general-purpose adaptations of the IBM Personal System/2 L40SX portable computer keyboard assembly. These should not be confused with the earlier buckling spring based IBM Space Saving Keyboard (SSK). The Model M4 was simply an L40SX keyboard assembly inside a dedicated case and attached to a PS/2 compatible controller, whereas the Model M4-1 adds a TrackPoint II pointing stick and two mouse buttons to the design. The Model M4s also had numeric keypad companion options whose design was developed from the Model M3 numeric keypad. M4s were used for space-restricted environments (as the name suggests), server monitor, sysadmin and laptop docking solutions. The M4-1 in particular has the distinction of being the first IBM keyboard with a TrackPoint pointing stick intended for desktop computer usage.


Marketing names
IBM Space Saver Keyboard, Lexmark Quiet Touch Keyboard, Unicomp Mighty Mouse
Lexmark, Key Tronic, Unicomp
M4 (without TrackPoint) or M4-1 (with TrackPoint)
Dye-sublimated (off-white) or pad-printed (black) PBT
Case dimensions
33cm x 6cm x 3.8cm (12.99" x 2.36" x 1.5")
Case material
Case colour(s)
off-white ("pearl white") or black ("raven black")
~0.68kg (1.5lbs)2
84 (ANSI) or 85 (ISO) key compacted tenkeyless
AT: 6-pin SDL to single PS/2 (M4) or dual PS/2 (M4-1) cable
Notable features
TrackPoint II (M4-1 only), PS/2 mouse passthrough (M4-1 only), detachable numeric keypad option
IBM Space Saving Keyboard (indirect)
IBM Space Saver II


The Model M4 design originated with the IBM PS/2 L40SX laptop's keyboard assembly, which was originally released on 26th March 19913. Development for the laptop started in February 1990, when IBM was already behind in the 386 laptop market. Looking to cut down development time for the machine, the buckling sleeves keyboard design skipped the usual mock-up and prototype stages and was quickly developed and pushed to production. IBM Information Products Corporation in Lexington, Kentucky under CEO Tom Hancock was responsible for the keyboard design4. Presumably, IBM decided to make a desktop version of its keyboard after the L40SX received good press for its keyboard5. The earliest year of manufacture for an M4 observed is 19926, at which point IBM Information Products Corporation had been spun off to form Lexmark International7.

Lexmark produced all M4s between 1992 and sometime between Q4 1995 and Q2 1996 and even marketed their own branded versions. In April 1996, Lexmark exited the keyboard market and all immediate production was transferred to Key Tronic. In 1998, Unicomp subsequently took over production and by the turn of the millennium had introduced their own branded version called the Mighty Mouse8. In November 1999, a "pearl white" Unicomp Mighty Mouse cost $99.00 USD whilst "raven black" one cost $109.00 USD9.

Various Unicomp Mighty Mouse<sup>1</sup>
Various Unicomp Mighty Mouse1

However, Unicomp continued to produce M4s for IBM until at least 200210. IBM M4s produced by Unicomp can be identified easily by the lock-light overlay style, which now uses Unicomp's signature lock icons instead of text. The Unicomp Mighty Mouse with TrackPoint was retired in 2008 and the one without retired in 2010. One unusual fact regarding Unicomp-made pointing stick Mighty Mouse keyboards was that Unicomp kept using a strain gauge TrackPoint II pointing stick instead of turning to their FSR stick design11.


The keyboard assembly design is exactly the same as it was for the keyboard used in the IBM PS/2 L40SX and CL57SX portable computers. It's an 84-key or 85-key tenkeyless keyboard with easily recognisable 1-unit modifier keys and square-profile F and navigation keys. The key arrangement is also more compacted compared to a typical TKL keyboard. The wedge-profile case is made from a PC + ABS blend and was available in off-white (what Unicomp calls "pearl white") or black ("raven black"). White models have dye-sublimated PBT keycaps just like the L40SX, however, the black version uses pad-printed PBT keycaps. M4s have flip-out feet typically found with rubber on the end to provide extra traction. In terms of branding, IBM branded models had a mix of raised and flat oval badges:

Unicomp Mighty Mouse keyboards left the oval badge blanked, and instead featured Unicomp branding on the lock-light overlay itself.

All M4/M4-1 keyboards are AT-compatible devices and have modular cables, typically a 2.5-metre1 6-pin SDL to PS/2 mini-DIN cable. As such, they're fully compatible with the SDL-based cables buckling spring Model Ms use and you could use a full-DIN AT cable with them if you so desired. M4-1s require a Y-split style dual PS/2 cable, however, single plug PS/2 or AT DIN cables will still work albeit with no pointing device functionality. Besides the SDL jack, all M4 and M4-1 keyboards also have a modular 10P10C jack for connecting the numeric keypad option to the keyboard. The keyboard acts as the controller for the numeric keypad option, thus the keypad alone cannot be used by itself without constructing a custom controller for it. M4-1 keyboards also have a pass-through PS/2 port for connecting an external PS/2 mouse to the keyboard.

All M4s use the original rod-actuated variant of IBM buckling rubber sleeve switches. Buckling rubber sleeves were IBM's primary portable computer switch technology in the first half of the 1990s. Unlike generic rubber dome switches or scissor-switches as you might expect from a keyboard like this, the rubber component in buckling sleeves plays no part in actually pressing down on the membrane, instead offloading this duty to rods on the keycaps that descend through the keyboard's switch barrels. This aims to reduce the mushy feeling of bottoming out on rubber dome keyboards since the actuation interface is more solid, whilst still keeping the design tactile and quiet. One thing to note is that the colour and apparent thickness of the sleeves themselves could vary from one example of an M4 to another. Overall, it should not make a difference in overall key feel.

The keyboard assembly itself (again, shared with the L40SX) takes a lot of inspiration from existing Model Ms in that it's a three-layer assembly complete with a barrel plate, membrane and metal backplate. The membrane's flex cables also socket into 2.54mm pitch TRIOMATE connectors just like its larger brethren. However, the way the assembly is held together is different. Instead of melted plastic rivets, the M4 assembly design uses hatching 'teeth' rivets instead. This means the main Achilles' heel of most larger Model Ms is eliminated and the assembly can be dissembled non-destructively (no need to do a screw or bolt mod on reassembly). On the bottom of the keyboard in the top-left corner, the lock-light LED daughterboard can be found inside its own compartment.


Model M4 keyboard

P/N 1397451 Lexmark-made M4<sup>14</sup>
P/N 1397451 Lexmark-made M414

The Model M4 proper was a straight adaptation of the Model M3-based IBM PS/2 L40SX keyboard assembly albeit in its own case and sporting a PS/2-compatible controller and LED lock-lights. IBM-branded examples have so far only been observed in beige/off-white - since they're made from PC + ABS, it is possible for them to visibly yellow. M4s seem to be far less common than M4-1s - it's unclear if they indeed sold less during their heyday or they were simply discarded more often since they lack the novelty of a TrackPoint pointing stick.

Model M4-1 keyboard

P/N 84G2524 Unicomp-made M4-1<sup>13</sup>
P/N 84G2524 Unicomp-made M4-113

The Model M4-1 was the more iconic variant of the family, being largely the same as the M4 but sports a TrackPoint II pointing stick and two mouse buttons. M4-1s can also be found in black, giving them a resemblance to ThinkPad portable computer keyboards such as their close cousins of the Model M6 family. Indeed, M4-1 part numbers can often be found in many 1990s ThinkPad hardware maintenance manuals as an accessory option. That said, beige/off-white versions also exist. Black M4-1s also lack dye-sublimated keycaps, meaning their key legends can wear off.


The Model M4 family was directly succeeded by the NMB-made IBM Space Saver II that was introduced in 1999. Better known by its model number RT3200, the Space Saver II was a full-travel rubber dome over membrane keyboard that featured a more conventional tenkeyless layout with Windows keys and larger modifiers and an SSK-style overlay numeric keypad. The Space Saver II could only be had with a TrackPoint stick. However, the stick is now a TrackPoint IV revision that was far more performant over the M4-1's TrackPoint II stick due to the inclusion of a negative inertia transfer function in its firmware. The design also received a third mouse button.

Further reading & resources


  1. Unicomp - The Mighty Mouse 84 Key [accessed 2021-12-31].
  2. Unicomp - The Mighty Mouse Keyboard [accessed 2022-01-21].
  3. IBM - THE IBM PS/2 MODEL L40 SX SYSTEM AND FEATURES [accessed 2021-12-31].
  4. Computerworld - 1 April 1991 [accessed 2021-12-31].
  5. SharktasticA - Revealed: IBM Model M3 & M6 laptop buckling sleeves keyboards [accessed 2021-12-31].
  6. SharktasticA - P/N 1398051 (1992, Lexmark) [accessed 2022-01-08].
  7. SharktasticA - IBM Information Products Corporation [accessed 2021-12-31].
  8. Unicomp - Unicomp Product Catalog (2000) [accessed 2021-13-31].
  9. Unicomp - Unicomp Keyboards [accessed 2021-01-02].
  10. Geekhack - Re: Unusual mini Unicomp Model M (98U0176), $24.99+shipping [accessed 2021-12-31].
  11. Deskthority wiki - Unicomp Keyboards [accessed 2022-01-01].
  12. SharktasticA - Revealed: The Story of the Model M4 family [accessed 2022-01-02].
  13. SharktasticA - SNKB-M1999-41K-83 [accessed 2021-01-08].
  14. SharktasticA - SNKB-M1996-4KB-83 [accessed 2021-01-08].