Si-Fe-Ti-Ne
The first thing I will focus on is how Si-Fe works in tandem with each other. This will help us to understand what seperates them from Si-Te.
The second function in a stack is called the auxiliary, or support, function. It helps you balance between extraversion and introversion, so we can make balanced decisions. Here, our auxiliary function is Fe, which makes ISFJ's skilled at reading other people and accessing their needs. Fe helps Si to shake off the rigidity that they can get when they become too reliant on the past that they don't consider any new opinions or options by making Si open up to other perspectives by people other than themselves. Auxiliary Fe helps ISFJ's understand why it is so important to consider other people in their decision making, tapping into their empathy and relating to their need for social harmony. However, if too much attention is placed on Fe, they run the risk of becoming a people pleaser with no boundaries. This will lead to difficulty forming healthy relationships and being walked over by others who are not so kind.
The third function in the stack is called the tertiary function. In ISFJ's, the tertiary function is Ti (introverted thinking.) In childhood and adolescence, we primarily utilize our first 2 functions, leaving out last 2 functions relatively undeveloped. ISFJ's usually use Ti in tandem with their Fe. A consequence of tertiary Ti is its ability to make harsh generalizations based on spotty assumptions. This can lead to a narcissistic worldview, and can also lead to tunnel vision where you cannot understand why somebody would believe differently than you do. Since the tertiary function is the child-like function, I believe that tertiary Ti can make one annoyingly pedantic, splitting hairs that don't need to be split. I also believe that it can make one a perfectionist. Watch out for the Si-Ti loop, where you over-analyze the past and crtitique every single mistake that you have ever made. To get out of this loop, you need to use Fe to check in with others to get an outside perspective, and in general connecting with people will help.
The fourth and final function is called the inferior function. In this case, extraverted intuition is the inferior function with ISFJ's and ISTJ's. Our inferior function is the least developed and the hardest one to use. Usually, ISFJ's seek new perspectives via the use of auxiliary Fe, so it's safe to assume that they don't have the healthiest use of Ne. Inferior Ne manifests in a couple of different ways. One way inferior Ne manifests is making you suddenly wanting to try some new idea out of nowhere (impulsivity,) but you stick to that new idea no matter what. This can be a problem because the inferior Ne can also stop you from seeing the consequences of that idea. Another way inferior Ne manifests is by catastrophizing, only focusing on the worst case scenarios and not the positive. It has been noted that ISFJ's are likely to 'snap' when dealing with people who propose suggestions that are irrational and out of touch with reality. This is actually due to inferior Ne. Snapping can also occur when one overworks themselves, and allows themselves to be treated like a doormat. On a more positive note, it has been said that Ne in ISFJ's is firmly used as a creative function-where they pursue dreams they deem as unrealistic through creativity and living vicariously through others, instead of living mainly through it. Last, but not least...in order to try new things, they have to reconcile their dominant Si with their inferior Ne. For example, they may have to get rid of prejudices they have from their time growing up and has a significance in their inner memories. When inferior Ne is developed, they can either become more free-spirited or completely reject all Ne-high personalities as foolish.
Si-Te-Fi-Ne
Unlike ISFJ, ISTJ has the Fi/Te function axis instead of Fe/Ti.
Auxiliary Te, like auxiliary Fe, works to turn dominant Si's attention towards the outside world. However, Te instead focuses on systems of hierarchy and objective standards. These objective standards help ISTJ's to logically understand why it is so important to consider different perspectives. It also helps them to make better judgements by obtaining important information such as standards, to adapt to increasing demands, and create a higher level of knowledge so that, again, they can adapt better to situations.
Since I already touched on inferior Ne, lastly I will be focusing on tertiary Fi. Like I said before, the tertiary function is a very immature and childlike function. Tertiary Fi manifests in these users being steadfast in their personal convictions, to the point where they cannot understand why anybody would not value the same things that they do. They use their Si as a database to create their Fi convictions from.