Says the Rambam [Maaser Sheni 10-1] :
כל שהוא חייב בערלה יש לו רבעי וכל שפטור מן הערלה אינו חייב ברבעי שנאמר שלש שנים יהיה לכם ערלים וגו' ובשנה הרביעית:
[The restrictions of] neta reva'i applies to all [plants to which] the prohibition of orlah applies. And all [plants] that are exempt from orlah are not obligated for neta reva'i, as [Leviticus 19:23-24] states: "For three years [your plants] will be orlah.... In the fourth year".
The Kesef Mishna says [and the Rashba as well in the Tshuvos 3-231] that the Rambam is talking only about ארץ ישראל but in חוץ לארץ there is ערלה but there is no רבעי as the Rambam says in Hilchos Maaser Sheni [9-1] and in Maachalos Asuros [Chapter 10].
That is a difficult pill to swallow since the Rambam here writes a general rule and mentions NO exceptions implying that it would apply EVERYWHERE.
Not only that, but the Rambam writes that this din emerges from the pasuk. Where do we see in the pasuk that the din applies only in ארץ ישראל?
Moreover, this din is already taught in the Yerushalmi [Orlah 1-1] where it says that if one plants a tree for three years with the intention that it should be a fence and then decides in the 4th year that he wants the fruits, there is no din of נטע רבעי because of the rule that if there is no ערלה there is also no רבעי. But the Yerushalmi doesn't cite the pasuk cited by the Rambam because this rule cannot be derived from the pasuk as we explained - for if we say [see Brachos 35 quoted earlier] that only a כרם is obligated in רבעי it must be that רבעי and ערלה are not interconnected [because in most fields there is NO דין רבעי even though there IS a דין ערלה]??!! So unlike the ruling of the Rambam "כל שהוא חייב בערלה יש לו רבעי", sometimes there is no דין רבעי even though there is a דין ערלה! How then does the Rambam derive a rule from a pasuk when this rule doesn't jive with a prominent opinion in the Gemara? And when the Yerushalmi itself neglected to quote this pasuk??
We also have to understand where the source of the Rambam is to say that whenever there is a obligation of ערלה there is also an obligation of רבעי. All it says in the Yerushalmi is that if there is no ערלה there is also no רבעי but not the converse - that if there is ערלה there is necessarily also רבעי? [See Derech Emunah who discusses this].
We also have to understand the סדר of the Rambam, for in the 9th perek of Hilchos Maaser Sheni the Rambam teaches דיני רבעי and in the 10th דיני ערלה. Apparently these dinim that "[the restrictions of] neta reva'i applies to all [plants to which] the prohibition of orlah applies. And all [plants] that are exempt from orlah are not obligated for neta reva'i" should have been written in the 9th perek in the context of דיני רבעי [because they are teaching when the laws of רבעי apply].
In addition, in the 10th perek of Maachalos Asuros the Rambam first wrote the dinim of ערלה and only then the dinim of רבעי. Why does the Rambam change the order from what he writes in Hilchos Maaser Sheni?
In short, this Rambam is כולו מקשה אחת - VERY difficult to understand!
[עפ"י דברי מו"ר הגאון הגדול רבי ד"י מן זצ"ל]
