The Zera Avraham [סימן י"ד אות כ"ב] discusses the איסור רבעי and suggests that it is really an extension of the איסור ערלה which is permitted if redeemed [see there]. The Derech Emunah [Orlah 9-1] also talks about this and notes the opinion of the Behag who only enumerates the mitzva of ערלה and doesn't enumerate the mitzva of נטע רבעי which would imply that they are one mitzva. He also quotes the Ramban [Rosh Hashana 10a] who writes regarding נטע רבעי in חוץ לארץ, that from the fact that the mishna says that "any mitzva that depends on the land is only kept in the Land [Eretz Yisrael] except for Orlah" and רבעי is not mentioned, even though רבעי is an independent mitzva, we see that רבעי is not obligatory in חוץ לארץ [see there at length]. This would imply that ערלה and רבעי are two distinct concepts, as the Ramban noted that רבעי is an independent mitzva.
Based on what we said earlier, the correct understanding is not that the איסור רבעי is an extension of the איסור ערלה which is permitted when redeemed, but the opposite - the איסור ערלה is a result of the איסור רבעי. Since one should eat his first fruits in front of Hashem in the fourth year, it is forbidden to eat for the first three years [as we saw in the Ramban Al Hatorah]. Therefore the Behag correctly included them in the same mitzva since they are conceptually related. But as we said, there is also an aspect of the mitzva of ערלה that is NOT connected to the mitzva of רבעי and that is what the Ramban [Rosh Hashana 10a] meant when he said that רבעי is an independent mitzva. So when the mishna mentions ערלה being in חו"ל that is not a proof that the same applies to רבעי because maybe this din stems from the aspect of ערלה that is unrelated to רבעי. So ערלה is both connected conceptually to רבעי and also independent.
[עפ"י דברי מו"ר הגאון הגדול רבי ד"י מן זצ"ל]
