So if you ask anyone what is like the “golden number” of the Seder night, they would say 4 (as in 4 cups, 4 sons, 4 questions by mah nishtanah...). But I heard someone once say that it’s all kinda a facade and every 4 is really 5- by the sons the fifth one is the son who never even showed up, by the cups we have the כוס אליהו, by the questions there’s that extra question in the Gemara...so does anyone know anyone or any good articles that expand on this idea?
You've got some great comments on your OP post identifying other 5s or 4/5 ambiguities. If you count the question about korbanot (Pesachim 116a) there are in fact 5, one is "missing." (I assume this is what @Clint Eastwood meant). @Clint Eastwood also makes a good point about the ambiguity of whether there were 4x or 5x the plagues at the sea. I have heard the idea of the "missing son," but don't know how old it is. The cup of Elija is actually based on a girsaot that say Rabbi Tarfon adds a fifth cup to the four, so there's actually a strong concept of a hidden "fifth cup" there. This article cited in a comment explains the origins of Elija's cup. There is a good article here and Rabbi Kasher zal has a kuntres on it and I think it's discussed in the haggadah of the Encyclopedia Talmudit. And even if there isn't a fifth cup, then there is a fifth expression of geulah not represented by a cup.
I cannot give you an answer that is specific to the haggadah, but I can tell you that the idea of "four that are really five" is a very common theme in kabbalistic thought. For example, in the introduction to Tolaat Yaakov the author describes how Hashem created an initial world of "Adam Kadmon" (probably similar to "Ein Sof") and from there the "four worlds" emendated. You will also see in many places that the four letters of God's name can really be broken into the 4 plus the tip of the yod, which represents the unity or invisibility of ein sof. The Shaarei Kedusha of Rav Chaim Vitale talks about this idea and it's also discussed in more recent "Chassidus" sefarim like Siftein Chein and Emunat Eitekha. Whether we are seeing kabbalah influencing the Haggada, both of them reflecting a common tradition, or coincidence is beyond me.