The Gemara 2a brings the statement of Rav Yochanan that there are 3 Maftechos, or keys which HKBH Does not give to a Shaliach, then on 2b it brings the opinion "B'maarava" That there are 4 keys.
B'maarava is usually understood to mean in Eretz Yisroel, but Rav Yochanan was also in Eretz Yisroel, so does it have another connotation in this case? Was there another academy in Eretz Yisroel other than that of Rav Yochanan to which B'marava refers?
The Masores Hashas in the Oz V'hadar Shas here refers us to a Targum Yerushalmi which also refers to 4 keys; not sure if this is relevant to this question.
Just because R' Yochanan says 3 doesn't stop others in Eretz Yisroel to say 4, maybe even in the same Yeshiva.
Rav Halperin in Atlas Etz Chaim refers to 5 generations of Amoraim in Eretz Yisroel. R' Yochanan is called 2nd generation. There were many ,many others. See there at length.
All the best, Reuven Weiner
The Gemara in Sanhedrin 17b explains that when the phrase 'Amrei b'Ma'arava' appears, the statement in question can be attributed to Rebbi Yirmeyah (who started off in Bavel and went to learn in Eretz Yisrael, Kesuvos 75a, but apparently maintained contact with his old study partners in Bavel). If so, it is possible that 'b'Ma'arava Amrei' is also referring to Rebbi Yirmeyah.
Rebbi Yirmeyah lived, and is buried, in Teveryah (as was Rebbi Yochanan) about a generation after Rebbi Yochanan.
M. Kornfeld, Kollel Iyun Hadaf