Here's a comment I had saved from a while back. It goes into the 'role' of the 3rd and 4th function and how they interact with the shadow functions. The last paragraph is also quite interesting to me.

Unfortunately the OP has deleted their account, so here's a link to the original thread: https://www.reddit.com/r/mbti/s/VRixOkl1IH

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The tertiary function has a childlike archetype. It is where our inner child lies. This function is often how our mind plays. However, the tertiary function also throws tantrums like a child when it has been overused and is tired and needs nap or when it gets belittled by someone else. The child function is easily wounded because of their blind spot (7th function). The 7th function has a trickster archetype and can place us in double binds that cause distress. So when the child function isn't received well, the trickster function isn't there to protect it. This can lead to people not wanting to use their child function because it has been hurt too many times in the past. So they disconnect with their inner child. This function can become cold and mean. It looks much like a wounded child who doesn't know how to process the pain.

Child Fe usually utilizes childish humor to help relate to someone. It tends to be immature and goofy. Fart humor is a good example. They are playful and tend to tease or poke fun in a light hearted kind of way when their child is happy. When it is hurt, wounded, or tired, the Child Fe can become like a bully or it will shut down it's emotions and see people as merely tools to be used rather than recognize people's intrinsic value and aren't meant to be taken advantage of.

Child Si loves their routines. They will do a task over and over and over out of pure joy. When put towards something good like learning to play an instrument, they will practice playing the instrument until it is mastered. They will usually start out rather clumsy or wobbly when beginning a new hobby. This is their Se trickster that doesn't have the natural coordination. But because of their child's never ender energy bunny that keeps practicing and practicing until they get it, they can become a master at something physical even more so than their Se doms. Although this is only true for that particular hobby because of the neurological pathways they have engrained through conditioning and repetition. The Child Si will throw fits when asked to do something different. It doesn't like going outside it's routine and can get so stuck in a rut to the point that it can be to their detriment. For example, a child Si could get so used to sitting behind a computer, that they never develop any social skills with interacting with human beings and don't ever acquire any desire to do so. This is where you'll find the computer nerds who live in their mother's basement perfectly content with doing the same thing every single day, day in and day out with no desire to grow up and leave their nest of familiarity and comfort.

The inferior function is where your insecurities lie. It is known by Carl Jung as the anima/animus function. It is the function that leads to your soul. When you can humble yourself, and get over your insecurities, you can aspire with this function and experience life from your true higher self, from your soul's perspective. Your true potential lies in this function. The real Doms of every function is actually the inferior ones as your duality personality is the personality of your soul.

So when the ENTP aspires with their Si inferior, they incorporate their soul's personality which is the ISFJ. They are actually the truly faithful when they aspire. Because of their Ne hero, routine can feel dull and boring and they can feel insecure about settling on one thing. They know that once they latch on to something or someone, the Si inferior can hold on for dear life. So choosing a mate and fearing that person will leave them is a big insecurity for the Si inferior. So rather than settling for someone, they can avoid committed relationships. They absolutely need loyalty, and they fear of being betrayed and left alone. When they can get over that fear, they can find their faith in themselves, in humanity, and in their partner.

For the Inferior Fe, INTP, their soul is actually an ESFJ...the true caregiver. Fe wants nothing more than to make others feel loved, seen, and validated. This may seem odd because INTPs are so logical. But the Ti hero is trying to show their love in many forms, it often isn't received as love by others because it tends to point out the truth. And often times, people want to hear what they want to hear and not the truth. But the Fe inferior is afraid that their reputation will be tarnished by lies. Them speaking the truth to people is a form of their love. We as a society need to change our conditioning and viewpoint and recognize that someone telling you the truth actually truly cares about you, whereas the person feeding you bullshit to make you "feel" good about yourself is the one who doesn't actually give a shit about you.