What's the difference between "apat na araw na ang nakaraan"

No offense, but are you learning the grammar or merely translating word for word?

@Bituingmaykinang
Actually, I am interested in the lexical and compositional semantics of the Tagalog language.

There's really no subtle difference if that is what you are looking for. Both is used by native speakers to mean the same thing.A lot of times, you just really have to accept that it is what it is.

@banyaga29
the diff is the latter sounds more ... hmm how can I put this. It's how old people speak? Like that's more of the traditional way of saying the phrase. I'd hear it in an all-Tagalog skit or drama. I'm a native speaker, but I can tell you even Filipinos have trouble with that. Hahaha! I think it's a matter of preference... or which degree of formality you want to speak with.Hope that helps!