We are up to Mishnah 5 of chapter 4, and its subject is kerem reva'i (a fourth year vineyard). 'Reva'i' refers to the fruit of the fourth year, which comes after the three years of orlah during which the fruit is completely forbidden to eat. This fruit must be brought up to Jerusalem and eaten there; and if a person is unable to bring it up, he may redeem it onto coins, bring the coins up to Jerusalem and spend them there on food.
This law closely resembles the law of Maaser Sheni, and the question is to what extent reva'i is equivalent to Maaser Sheni. On this point Beis Shammai and Beis Hillel disagree: Beis Shammai hold that it resembles Maaser Sheni in certain matters but not in others, while Beis Hillel hold that Maaser Sheni and reva'i are halachically identical. The basis for this is a gezeirah shavah from the word "kodesh" that appears in both verses, which links them and establishes that the same laws that apply to Maaser Sheni apply to reva'i as well.
It is worth noting: the Mishnah uses the expression "kerem reva'i" because the Tannaim disagreed as to whether the law of reva'i applies only to grapes or to all types of trees. Halachically, one should read "neta reva'i" - the fruit of the fourth year of any tree.
The view of Beis Shammai - "it has no fifth and it has no biur":
The words of the Mishnah: "Kerem reva'i, Beis Shammai omrim: ein lo chomesh v'ein lo biur" - regarding a fourth year vineyard, Beis Shammai say: it has no fifth and it has no biur. Although reva'i resembles Maaser Sheni in several matters, it does not resemble it in these two laws:
Chomesh (the added fifth): One who redeems his own Maaser Sheni onto a coin must add a fifth. How so? If he had wine worth four, he transfers the sanctity from the wine onto coins worth four, and adds a fifth coin - an addition of twenty five percent. The Mishnah calls this a 'fifth' because for every four a fifth is added, and he brings the coins up to Jerusalem. This law applies only to one who redeems his own produce; but one who sells his wine to his fellow, so that the buyer drinks it and the seller takes the money and brings it up to Jerusalem, sells at the regular price without adding a fifth.
Biur (removal): Twice in the shemittah cycle, at the end of the first three years and at the end of the second three years, there is a final deadline for settling accounts: to separate all the terumos and maasros as required, and to bring the food up to Jerusalem and eat it there. One who did not manage to reach Jerusalem by the deadline must remove the Maaser Sheni from his possession, and this is biur. In practice, according to almost all the authorities, the deadlines are erev Pesach of the fourth year and of the seventh year.
The question therefore arises: if a person has reva'i produce, such as wine of the fourth year, and the time of biur that applies to the other maasros and gifts has arrived - is he obligated to remove it from his possession, to destroy it or to give it to someone else? According to Beis Shammai he is not obligated, since reva'i is different from Maaser Sheni, which does carry an obligation of biur.
Beis Hillel are stricter: "U'Veis Hillel omrim: yesh lo chomesh v'yesh lo biur" - and Beis Hillel say: it has a fifth and it has biur. He must add a fifth when redeeming, and he is also obligated in biur and removal from his possession. The basis for this is the verse regarding neta reva'i: "And in the fourth year all its fruit shall be holy, a praise to Hashem", and the verse regarding Maaser Sheni: "And every tithe of the land, of the seed of the land, of the fruit of the tree, is Hashem's; it is holy to Hashem". The word "kodesh" shared by both verses is what links them in a gezeirah shavah, and equates the law of reva'i with the law of Maaser Sheni.
The gifts to the poor in a fourth year vineyard:
The second part of the Mishnah deals with the gifts that are unique to a vineyard:
Peret: This is like leket in grain - one who harvests his grapes and one or two grapes fall from him leaves them for the poor.
Olelos: Small clusters that are misshapen, having neither a shoulder nor a dangling portion, and these too are given to the poor.
The question is what the law of these gifts is in a fourth year vineyard - whether they are left for the poor or not. The basis of the dispute: Beis Hillel hold that reva'i is entirely equivalent to Maaser Sheni, and they hold like Rabbi Meir that Maaser Sheni is the property of Heaven. The fruit does not belong to the farmer but, as it were, to the Holy One Blessed be He, and a person brings it up to Jerusalem and eats it there, as it were, together with the Holy One Blessed be He - but he is eating from what belongs to Above, and not from what is his own.
Since that is so, these fruits are exempt from the gifts to the poor, for the verse regarding ollelos and peret speaks of "karmecha" - your vineyard - and this produce is not his but belongs to the Holy One, blessed is He. Beis Shammai, by contrast, hold that revai is not like maaser sheni, and therefore the fruits are the owner's private property - ordinary money - and he is obligated to leave the gifts for the poor.
These are the words of the Mishnah: "Beis Shammai omrim: yesh lo peret v'yesh lo ollelos" - he must leave the single grape or the two grapes that fall during the harvest, and likewise the small, sparse clusters, for the poor, even in the fourth year. "V'ha'aniyim podin l'atzman" - and the poor redeem them for themselves.
The poor who gather the peret and the ollelos may not eat them outside Yerushalayim, since they are fruits of neta revai. Therefore they must either bring them up themselves and eat them in Yerushalayim, or redeem them onto coins and bring the coins up to Yerushalayim. It emerges that the poor do indeed gather the fruit, but it is restricted in their hands because of the sanctity of revai, and they are treated in this regard like anyone else.
"U'Beis Hillel omrim: kulah la'gas" - there is no need to leave anything for the poor from the fruits of the fourth year, for they are consecrated property and are not his to leave; rather, everything goes up to Yerushalayim. "Kulah la'gas" means that all the fruit, including what would have been peret and what would have been ollelos, is gathered into the gas - the winepress - is pressed and made into wine, and all of the wine is brought up to Yerushalayim and eaten there, without anything being set aside from it for the poor.