Whilst S and N are mental muscles for PERCEIVING information, T and F are mental muscles for JUDGING or making decisions. The third letter of your four-letter code shows which mental muscle you prefer to use when making decisions.
	Bad Joke: A man went to see a doctor, saying "I may have difficult making decisions". The doctor questioned him and the man dithered over every one. "Yes, you're indecisive" concluded the doctor. "OK", said the man, "but can you give me the medical term so I can tell my wife".