With the stronger program in the 4.0 (Steinitz) module came some useful extra features. On the right are the Grand Master Series 4.0 and Sargon 2.5 side by side. You can see the additional labels next to the buttons - options, verify, take back and clear. Options accesses a whole range of functions including - new game, clear board, cancel opening book, change side to move, easy mode, book practice mode, sound quieter and off, resign and black from the bottom.

Wiki puts the increase in chess playing strength at 100 Elo. The 4.0 is also a more pleasant responsive machine to play.
 

The Auto Response Board (ARB) was the first auto sensory chess computer to be produced. Released in January 1980 it appeared amazingly early in the commercial development of chess computers. With the Sargon 2.5 program the ARB was the top chess computer of its day. It cost £649 in the UK, the equivalent of £2,000 at today’s prices. However the ARB was not a particularly rare machine in the USA. The so called Grandmaster Series was perhaps less common. Most ARBs come with the Sargon 2.5 program and the other program modules produced, the 3.0, 3.5 and 4.0 modules are much rarer. My ARB with 4.0 is like a piece of well used antique furniture. It oozes quality and is a pleasure to play against. The Sargon 2.5 version is much less user friendly.

This ARB is normally stored in the huge mountain case you see above. However there is also a purpose made Applied Concepts leather carrying case, which itself is a rarely found accessory.

It is the combination of the Grand Master Series ARB, the 4.0 module and leather carrying case which just gets this chess computer into the Rare feature.