Hierarchy of Mental Functions. Each of the 16 personality types has a characteristic pattern in the alignment of the four mental functions. This pattern is referred to as a "hierarchy" because they typically differ in the degree of influence on the personality and the degree they are consciously experienced. As indicated above, the most important or influential function is termed the "Dominant" function (#1) and the second most important is termed the "Auxiliary" (#2). The third in the hierarchy is called the "Tertiary" (#3) and is the polar opposite of whatever function is the Auxiliary. The 4th in order Jung termed the "Inferior" function (#4). It is the mental process with the least conscious awareness and typically the least developed of the four functions. It is the polar opposite of whatever is the Dominant.
So if INtuition is Dominant, its opposite - Sensing - is the Inferior or 4th. If Feeling is Dominant, then its opposite - Thinking - is Inferior or 4th.
The rationale for the opposite relationship of the Dominant and the Inferior (fourth function) has to do with energy and the natural polarity of the mental functions. For example, a person with dominant Intuition will direct his/her primary energy to this function - which happens to be in the exact opposite direction of Sensing. It is like trying to go North and South at the same time. It is much easier to couple that dominant with either the Auxiliary or Tertiary because these are not polar opposites to the main direction. They are like East and West on the compass. So navigating NW or NE is a natural direction of movement -- but North-South is not.
You may be uncomfortable with using Jung's term for the 4th function: "Inferior." Bear in mind Jung was writing in the 1920's and his works had to be translated from German to English. Caution against jumping to conclusions on this Dominant-Inferior pairing. To wit, although a person's dominant function might be Feeling and therefore their inferior function is Thinking, do not presume that their "thinking" is inferior, i.e. defective!! There is often a built-in growth dynamic to consciously develop whatever is opposite one's dominant mental function.
Attitude is Also Important. Here's another Jungian term that carries a somewhat different meaning in the English speaking world. We don't mean your world view or the rosyness of your outlook. It isn't like "Lose the attitude, Dude!"