This is the 4MB RAM cartridge for the Sega Saturn that allowed the Saturn to perform arcade perfect ports that no other machine at the time could.  Well, except for the Neo Geo with it's own games.  Companies such as Capcom and SNK made may games compatible with the 4 MB cartridge as did a few other companies such as Data East and Atlus.  Unlike some 1MB RAM compatible  games which work with the 1MB cartridge missing, 4MB RAM Cartridge compatible games wont run at all unless the cartridge is present.  The cartridge normally came packaged with games but it was also available to buy separately for the same price as buying a game with in included.  Hmm, smart idea that wasn't it (-_-)
So, what games are compatible with the 4MB cartridge?  Well all of them are except for The King Of Fighters '95 which needs it's own special cartridge.  But what games will only work with the 4MB Cartridge plugged in?  Well read on to find out.

Vampire Saviour

X-Men vs. Street Fighter

Marvel Super Heroes vs. Street Fighter

Dungeons and Dragons Collection

Final Fight Revenge

The King Of Fighters '97

Street Fighter Zero 3

Cyberbots (choose between 1 or 3 MB modes)

Pocket Fighter (choose between 1 or 4 MB modes)

Marvel Super Heroes (choose between 1 or 3 MB modes)

Astra Super Stars