What is notable about type 4 PCMCIA cards?

Type 4 PCMCIA cards are notable because they are unofficial and very rare. This specific type of card was intended for specialized purposes, which means they did not achieve widespread adoption like the other types of PCMCIA cards (Type 1, Type 2, and Type 3). Type 4 cards could accommodate larger sizes and were envisioned for specific applications that did not see significant demand, resulting in their rarity in the market.

This rare status highlights the evolution of PCMCIA technology and how different card types were designed to fulfill various functional needs. In contrast, other types of PCMCIA cards have become more common due to their broader utility for things like storage, networking, and additional capabilities in devices. The other answers suggest widespread usage or specific functionalities that are more closely aligned with better-known PCMCIA card types.