An Airport can have several abbreviations, it all depends on who you ask

My home base airport's ICAO (that's the International Civil Aviation Organization ) code is ESGG
Every airport in Sweden has ES as the first 2 letters, and I believe every major airport in the US starts with a K (KORD for O'Hare, KLAX for Los Angeles etc). As a pilot you will use the ICAO codes.
But there is also the IATA (International Air Transport Association) code, and for my airport the code is GOT...but the IATA code is used by the bagage handlers, (amongst others). As the IATA codes are integrated into the computers that handles automated baggage sortation systems (you know the neat tag your suitcase gets when you travel)
I used to work as a ramp rat for 2 summers in a row

But to sum it up: use 4 letter abbreviations in FS
