OK, so I spoke hastily. I just get annoyed by the haziness most people use the X in the four letter code to represent. Each person is a four letter type, but there is some overlap that happens when you break each type down into its requisite 5 parts.
Extraverting
Initiating
Expressive
Gregarious
Active
Enthusiastic
Introverting
Receiving
Contained
Intimate
Reflective
Quiet
Sensing
Concrete
Realistic
Practical
Experiential
Traditional
Intuiting
Abstract
Imaginative
Conceptual
Theoretical
Original
Thinking
Logical
Reasonable
Questioning
Critical
Tough
Feeling
Empathetic
Compassionate
Accommodating
Accepting
Tender
Judging
Systematic
Planful
Early Starting
Scheduled
Methodical
Perceiving
Casual
Open-ended
Prompted
Spontaneous
Emergent
So yes, you still have your dominant letter because at least 3 out of 5 will fall on one side or the other.
As far as learning how to use a nondominant function, there's a problem of origin and avenue. Let me see if I can paint the picture in my head...
When a fresh brain starts getting nerve impulses through its brand new nervous system it doesn't know what it means. It has to figure it out through trial and error, and to accomplish that it needs to use a function through the brain to process it. How that function is selected it uncertain, but it becomes very, very important to that individual because it is now intertwined directly with how the body understands its surroundings. The neurons have literally become a model of that function however it is expressed. As the individual starts deciphering information it becomes obvious that the single function is insufficient to deal with all the stimuli, so the auxiliary function is built to handle the excess (now the person has both introverted and extraverted functions through which to build the rest of their psyche, but one is far more involved in the process of 'being').
When a person learns about the complexities of life and that each problem is solved best with a tailor made solution, different functions become available, but those functions are always on the outside. Understood by the dominant functions, just like math or language. You can use a function extensively enough to build a neural network to deal with that function, but in order for the person not to develop multiple personalities, information that runs through that network must reference the original dominant network that connects the brain to everything else.
When a person undergoes a stroke, parts of their neural network are shut off, or die entirely. That's why after a stroke people's personality sometimes changes. Introverted people become extraverts, judgers become perceivers... its like starting over because they've lost that foundational understanding of their body's nervous system.