Alternative Answer: The Gemara in Horiyos (Daf 6b) answers that 'All of them were said in one go' - meaning that all five Chata'os were learned in one context, and since one of them is not applicable be'Tibur, all of the others wer also learned not by a Tzibur.

Current Text: Whereas here the text reads - 'Resh Lakish said that 'Four were taught and they established them by five' ...

Clarification: Meaning that from Sinai they were given four Meisos and one Ro'eh, only during the days of mourning of Moshe they forgot which one, so they decreed that all of them must die.

Current Text: 'And if you would think that that speaks by a Tzibur, are they applicable to a Tzibur? We must therefore say that we learn what is hidden from what is revealed' ...

Clarification: It goes without saying that if the 'Halachah' by all of them had been that they should die, that they would all have been learned in one go - and since some of them are not applicable to a Tzibur, none of the others will apply to a Tzibur either ...

Clarification (cont.): But even now that the 'Halachah' states that some of them are Ro'eh, we will still say that all of them do not apply to a Tzibur ...

Clarification (cont.): And it goes without saying that if the one that is Ro'eh is among those that are not applicable to a Tzibur ...

Clarification (cont.): In which case the four that were taught must die were all said in one go, and all of them will not apply to a Tzibur ...

Reason: Seeing as at least two of them were taught not by a Tzibur, so too, will the other two not apply to a Tzibur ...

Clarification (cont.): But even if you will say that where the owner has received his atonement or where the year passed, one of them is the one that was taught 'Ro'eh', the other one will have been taught le'Misah, together with the three to which Tzibur is not applicable ...

Clarification (concl.): And the fourth one will not apply to the Tzibur either.

Conclusion: Consequently, the one that was taught Ro'eh, which they forgot and on which they decreed Misah, even if it applicable to a Tzibur, we will nevertheless learn Efshar from Efshar ...

Conclusion (cont.): And since the four were learned for Misah only by a Yachid but not by a Tzibur, since they were learned at the same time, the remaining one which was taught Ro'eh and on which they decreed Misah because it was forgotten, we will also decree only by a Yachid and not by a Tzibur ...

Reason: Seeing as we are learning Efshar from Efshar.

Clarification: On Sinai they were given one for Misah and four for Ro'eh, but during the days of mourning of Moshe they forgot which was which, so they decreed on all of them that they must die ...

Clarification (cont.): And the Gemara relies on the conclusion 'Just as what is explicit is confined to a Yachid ... ' (See Shitah Mekubetzes 24), only before it concludes, it queries his (Rebbi Nasan's) words, and then cites the conclusion of Rebbi Nasan.

Question: The Gemara asks 'Let us see how the Halachah was handed down, by a Tzibur or by a Yachid?' ...

Clarification: Why did they decree that all of them must die, bearing in mind that three of them are applicable to a Yachid and not to a Tzibur, and two of them, to a Tzibur as well ...

Clarification (cont.): So let us see whether the one one that was taught that it must die is one of the three, then we will decree Misah on the three, and on the two, Ro'eh ...

Clarification (concl.): Whereas if it is one of the two, they ought only to have decreed Misah on the two, and on the three, Re'iyah?

Answer: And the Gemara answers that they forgot two things - They forgot a. which one was taught for Misah, and b. whether it is from the two or from the three, and that explains why they decreed on all of them.

Clarification (cont.): And it is now that the Gemara concludes, and asks 'If you would think ... ' ? ...

Clarification (cont.): Thus is a proof that we learn what is hidden from what is revealed; Just as what is revealed ... ' ...

Clarification (concl.): With reference to the four that were taught to be Ro'eh, were all handed down in one go, what is hidden we now learn from what is revealed - just as Tosfos explained with regard to Resh Lakish (in the previous Dibur).