halacha - What happens to the obligation of rivai when a new fruit tree reaches its third or fourth year during Shmitah?

We learn in Leviticus 19:23, 24 that the fruit of a new tree is forbidden (orlah) for benefit for its first three years, but all of the fruit from the fourth year is sanctified (revai) and [when the Temple existed] would be brought to the Temple for the Kohanim. Since during shmitah or yovel, the fruit tree owner must declare the fruit that grows on his tree to be hefker (abandoned) and permit the citizens to take fruit to the extent of their personal needs. Accordingly, whould a tree be exempt from the mitzvah of revai if its fourth year fell on a shmitah or yovel year?

Also, since we do not have the Temple anymore, but we still observe shmitah and yovel (well at least some farmers do), what is the practice for dealing with the fruit of a four-year-old tree in a normal year? Is it considered hefker?

Answer

Rambam Maaser Sheni 9:7 (based on Mishna Maaser Sheni 5:1):

מי שהיה לו נטע רבעי בשנת השמיטה, שיד הכול שווה--צריך לציינו בקוזזות אדמה, כדי שיכירו בו, ולא יאכלו ממנו, עד שיפדו. ואם היה בתוך שני עורלה--מציינין אותו בחרסית, כדי שיפרשו ממנו: שאם ציינו בקוזזות אדמה שמא יתפרדו--שאיסור עורלה חמור הוא, שהיא אסורה בהניה. והצנועין היו מניחין את המעות בשנת השמיטה ואומרין, כל הנלקט מפירות רבעי אלו מחולל על המעות האלו--שהרי אי אפשר לפדותו במחובר, כמו שביארנו.‏
When a person had [an orchard that was in] its fourth year of growth in the Sabbatical year when everyone is allowed equal access to it, he must mark it with mounds of earth so that [those who take the produce] will recognize [its sacred quality] and not partake of it until they redeem it. If [the produce is] within the [three] orlah years,it should be marked with baked clay so that [people] will shun it. [We use clay instead of mounds of earth], lest the latter crumble. [The rationale is that] the prohibition of orlah is more severe, because benefit from it is forbidden.
Those who are meticulous in their observance of the Torah's prohibitions would set aside money in the Sabbatical year and say: "The holiness of everything harvested from these fruits of the fourth year is transferred to this money." [The redemption must be carried out in this fashion,] because it is forbidden to redeem [the produce] while it is attached to the ground, as explained. (translation from here)