Hm? This isn’t a matter of “one in certain situations/for certain subjects, one for another.” They’re different grammatical constructions for different purposes. It’s trying to illustrate the difference between modifying nouns with set prepositional expressions and modifying them with verbs.
In 3, we have two complete clauses using the prepositional expressions “as” (として) and “about” (について), which are then used as modifiers on the other side of the arrow (not complete clauses).
“That bus is used to commute to school” (lit. “This bus is used as a means of school transport”) becomes “A bus used as school transport.” (In English I would probably translate this to “a bus used to commute to school” in this scenario, but I’m trying to illustrate the grammar transition here more literally.)
“(I’ve) written a book about life” becomes, in this example “A book about life” as a subject.
In both examples it’s showing you that you can use the の particle to attach these prepositional phrases–として and について – to a noun as a modifier. This is pretty convenient and simple.
In 4, it’s showing you how verbs/predicates can be attached to nouns as modifiers instead. In this case, “That bus runs by city management” (overly literal translation for grammar) becomes “A bus run by the city,” and “(I) express opposition to Plan A” becomes “Opposition to plan A.” In both cases, the verb is simply appended to the front of the noun in its plain form, and that’s the grammatical point it’s trying to illustrate.
If you’re asking about the different selections in vocabulary here, it’s just selecting differently nuanced ways of saying something–with both 3 and 4 involving spins on an “as/for the purpose of” and “about/regarding” situation–to show the differences in grammar depending on whether you’re working with a prepositional phrase or a verb.
Set prepositional expressions like について and として can modify nouns by being attached with の, while other verbs get attached to the front in their plain form. Two different ways of modifying nouns depending on what part of language you’re working with.
*(Technically について and として are verbs too, but they’re used as prepositional expressions and fall into their own usage case for this grammar, as far as I’m aware; don’t get hung up on that; just wanted to point it out in case someone was ready to pounce on it.)
Hopefully I understood your question here, and sorry if I was off-base!
EDIT – Oh, sorry. I see. You’re asking why those two take the の particle while 対して and よって, also て-form verbs, just get reverted to their plain form. It’s so natural for me to partition off について and として now that I didn’t even think about that. It is indeed something you just have to remember, but I can’t think of any others in N3 study that require that exception. If you look those two up on jisho.org or another source, they’ll be tagged differently from other verbs as “expressions” too. Helps to just think of them a prepositions rather than verbs.
EDIT EDIT — Oh, this thread is also a year old. Rip me. Assuming you solved this one.