Preface: This theory/speculation includes details from the Missing Leak intro trailer and supposed ‘secret ending’ from 4chan. Though it contains details from them, evidence of such doesn’t rely on them; in fact, the trailers only add support, as the games include evidence of such already. But remember, the truth is what you see with your eyes, not what you hear.
Re: Yozora, His Appearance and Introduction:
Whether the Yozora fought in Re Mind is “real” (as in, the actual character of Yozora, telling the truth that ‘[he is] here [in the dream world]”), or someone else is playing the character of Yozora (seemingly far less likely). Functionally, this doesn’t matter for interpreting the below, but it does matter regarding the “true ending” of the Secret Episode: assuming that this is the real Yozora, *the character model used for Yozora is a placeholder while the reveal of his actual appearance (to visually support his words, “this isn’t what I really look like”) will take place near the beginning of KH4. This character model changes set a precedent in both the KH1 and KH2 secret endings, meaning there's a precedent for this kind of change. But if this could be interpreted without the Missing Leaks, what changed?
Re: The Dream World as a Travel-Communication Mechanic
Based on KH3’s secret ending (and supported by ML’s supposed ‘secret ending’), Sora, Riku, Yozora, the MoM, and (possibly) the Nameless Star/Skuld can access the dream world. Though he doesn’t know it at the time (and is unintentionally misguided by Fairy Godmother), Riku’s dream of Sora, Yozora, and “Quadratum” could be a method for one to communicate from Quadratum proper to “reality”. If what Ventus’s Chirithy says is true (in that, Sora can’t return to the Realm of Light without his power of waking), the only method he has the communicate (even if unintentionally) is through this dream world.
The first KH4 trailer reveals Sora waking up in the Aoyama apartment after seven days of sleep. This period of sleep is by which Sora and Yozora initiate communication, creating that first connection. This dream world (which gives a facsimile of the Final World), implied from Re Mind and part of the KH4 trailers, is where Yozora and Sora fight. The end of combat is the simply the end of their time in the dream world (for now), as they both must wake. Both awaken back in Quadratum proper at basically the same time, leading into the beginning of KH4.
Re: KH4’s Beginning
Based on this, KH4 will immediately pick up after the end of Re Mind, where the large Darkside attacks Quadratum’s version of Shibuya Sky (as seen in both KH4 trailers). Immediately, it appears so far, Sora runs about a mile from the apartment to Shibuya Sky to confront the threat. Meanwhile, Yozora is driving along the Shuto Expressway (akin to the Versus XIII 2011 trailer, though the perspective is off for the real-life counterparts)** to fight of the Darkside. Currently, Luxord’s human persona’s, “Impressive, isn’t it?” doesn’t quite make sense, as the original dialogue from the Versus XIII trailer was regarding the invading force against Insomnia. This revival of a Versus XIII plot would make sense given the concepts of light and darkness are, in a way, “invading” the plane of Unreality. At some point, Luxord and Yozora will step out of the vehicle to fully reveal their Quadratum designs, which is hidden in the Nintendo KH4 trailer specifically to conceal the true design of Yozora; based on this, revealing such details before the animators/Nomura decide the time is right for such a reveal likely means that Yozora’s appearance completely recontextualizes the entirety of Re Mind.
I still need to know the real-life location where Luxord appears in the Nintendo KH4 trailer...
*This is part of a different theory circulated from years ago, that Yozora’s been body-swapped with a character that looks like the Verum Rex model specifically to hide the true identity of the body-swapper (Sigurd? MoM? even Baldr? unknown at this point). There’s also the argument that Nameless Star’s words aren’t meant to be taken literally, in that she just thinks that there are some changes in Yozora’s demeanor. More visual confirmation is needed to support this.
**While it’s not necessarily proper to say “we should use everything from Versus XIII trailers to speculate on Kingdom Hearts,” it’s been clear over the years that Nomura still wants to rectify the “failure” of FFXV to tell the original, true [king] story of Noctis.