The Erune-Tingle-Zelda Postulate is my humorous title for canonical evidence that, in-universe, the Links not only look exactly the same (aside from their clothing colors) but also sound exactly the same. This, of course, contradicts The Legend of Zelda: Four Swords Adventures, so I wish to present my thoughts here in the hopes that it can be accepted as one of those things that differ between the manga and the game. In doing so, I will also present evidence against my belief for the sake of being thorough.
"The Erune-Tingle-Zelda Postulate states that because Erune, Tingle, and Zelda failed to comment on any difference in the Links' appearances or voices, and because they either implied or outright commented that the Links' appearances and voices are identical, the Links' appearances and voices in-universe must be identical or else so similar that any differences are undetectable at baseline awareness." - Socrates, 390 BC
Let me explain further. When Erune meets the Links, she outright tells them that they have the same face and asks if they are quadruplets. This means the Links not only look identical but also sound identical - common phenomena in identical siblings, especially when they are children. To further eludicate this, consider Tingle's interactions with Vio in the Tower of Flames. When he spots Vio about to break the Dark Mirror and confuses him for Green, the only thing he notes that has changed about "Green" is his clothing color. He never says that "Green's" voice changed in any way, and he is completely convinced that Vio is the exact same being as Green. This means, once again, that the Links look identical and sound identical in-universe. As a final example, we look to Zelda after she has been rescued from the Dark Cloud at the Palace of Winds. When evaluating the Links standing around her, she is unable to tell them apart and wonders which one is "the real Link". Though this panel is treated as a joke, I firmly believe the core of the joke is not relevant to the point I am trying to make here, and so I present it as further evidence that the Links look and sound identical. Zelda, who knows Link best, is unable to tell them apart and does not comment on any of them seeing to deviate greatly from Link in terms of appearance or voice. This is perhaps some of the strongest evidence, as Zelda knows Link very well and would likely comment on things that differentiate the Links from him in appearance or voice - yet she says nothing of the sort even after they leave the Palace of Winds.
It would be intellectual dishonesty, carelessness, and laziness not to address counterarguments to my beliefs. An easy counterargument is simply, "Those characters didn't mention any differences between the Links because it wasn't relevant." This could certaintly be true. A similar counterargument is that I am twisting the words and actions of the characters - particularly Tingle and Zelda - to fit my agenda, and I can understand the reasoning behind this. For those characters specifically, some implications must be extrapolated from their words/actions if they were to be used as evidence for the Erune-Tingle-Zelda Postulate given that they were not direct, literal proclamations of the Links being identical.
There is, however, a more crucial piece of evidence which is more obscure but more powerful. In the magazine version of the first two chapters of the manga, the Links each have a different font for their speech bubbles. This is no longer present after chapter two of the magazine run, and it was not retained in the tankoubon releases. I personally believe that Himekawa initially wanted the Links to have different voices because of the Links' voices in The Legend of Zelda: Four Swords Adventures but later scrapped the idea when it became too much of a hassle or when it became difficult for the intended audience to read. It is a shame this was not retained, as it makes ambiguous speech bubbles less ambiguous in terms of who the speaker is. Regardless, this evidence can still be used to argue against the postulate, and so I offer it here.
I will put photos showing the different fonts on this page as soon as I find a copy of the magazine issue with chapter one, as I only have the magazine issue with chapter two as of now.
Why does this postulate matter? Well, mainly, it means I can justify my dislike of people making designs for the Links where they differ visually from each other in significant ways. (People drawing the Links with different-colored eyes is my biggest pet peeve.) It also can be used as justification for if, say, a person wants to voice a project related to the manga and wants to voice all the Links themselves rather than try to find a different person to voice each one. Finally, I feel that driving home the point that the Links are identical would remind more of the fanbase that the Links are all just different but equally valid versions of Link. They are all Link. Every one of them is Link. I beg you all to stop trying to make them into their own separate people just so you can ship them because you're too cowardly to admit you enjoy incest or selfcest.