I have asked through the app Hello Talk the meaning of the word 대 in this picture and I got a very kind girl (Eunyoung 은영) who told me it is probably a 의존명사 which I later found to be called "dependent noun" but didn't find this to be a concept in other languages other than Korean language. If someone could explain their thoughts on this subject I would be very thankful in English or Korean. What is a dependent noun? And Is 대 a dependent noun? How would you translate 대 to English?
Nouns that cannot exist by its own, but has to follow some specific grammatical rule.
Example: 지(Noun of uncertainty) can only be used with modifiers(i.e. describing this noun with a verb or adjective).
상황이 어떤지에 따라 달라질 수 있어. (상황이 어떻다 + 지[dependent noun] + 에 따라 달라질 수 있다)
(Things could be different depending on how the situation is.)
그가 숙제를 한 지 몰랐는데.
(I don't know IF he has finished homework.)
Example: 수(ability) is only used for ㄹ/을 수 있다/없다.
대 is sometimes a dependent noun as it is also a counter(수사). It has to follow the rule (noun+number+counter) or (number+counter+noun). A number has to be before a counter.
차 4대 -> 4 cars(대 is the counter for cars)
4대 -> 4 generations
But 대 in your example 4대 위협 is not a dependent noun at all. It is the hanja of 大, meaning big.
To my knowledge, 대 is not a dependent (aka: bound) noun. 대로, 데, 때, and 때문 are though.
Anyways, such a noun cannot be used alone, it must be preceded by a word that describes it such as an adnominal (formed from a verb root with (으)ㄹ, 는, (으)ㄴ, (었)던, etc.), a prenoun (such as 그, 이, 저, etc), among other possibilities.