He started by designing a new system to replace a four-letter code used by military attachés that had been in use since around 1918.

It contains four-letter codes that denote variants of languages.

All trains whose four-letter codes (indicated both on the train and on the light board) begin with the same letter share the same terminus.

Each former country name in ISO 3166-3 is assigned a four-letter alphabetic code.

Each script is given both a four-letter code and a numeric one.

That made the major assumption that alien life, like life in the solar system, would be built around a four-letter molecular code.

We are all transient patterns of information, passing on the recipe for our basic forms to future generations using a four-letter digital code called DNA.

The encoder converts the four-letter code into four designated audio tones.

And it can be sequenced - its four-letter code "read" - by the same methods.

Each entry is assigned a four-letter alphabetic code, where the first two letters are the deleted alpha-2 code.