Why is it called Berachah Me'Ein Shalosh, and not Arba'?

Question

Why is the blessing recited after eating the 7 Species of Israel called Berachah Me'Ein Shalosh, (one that is equivalent to three) and not Berachah Me'Ein Arba' (one that is equivalent to four)?

The idea, as I understand it, is that these species kind of deserve a proper Birkath HaMazon, but this is not a proper meal, so we say a Berachah that incorporates the themes of all the Berachoth in Birkath HaMazon.

If this Berachah only incorporated the themes of the first three Berachoth, which might make sense because those fulfill a biblical requirement, whereas the fourth Berachah is rabbinic, then I'd see it as a reflection of that parallelism.

But that's not the case. It has the elements of all four Berachoth. So why do we pretend it doesn't?

Answer 1

The truth is that not all agree it should be said. The Avudarham (to the Bracha of Mezonot) provides a nice summary of the opinions:

כי אל טוב ומטיב אתה מעין ברכת הטוב והמטיב. יש אומרים שאין לומר כי אל טוב ומטיב אתה שלא נתקנה אלא מעין ג'. ויש אומרים שצריך לאומרו שאע"פ שנתקנה מעין ד' אין קורין אותה אלא מעין ג' שהם מן התורה. ויש אומרים שאין לאומרו בברכה של ה' המינין ולא בברכת הפירות שהוא על העץ ועל פה"ע אלא בברכת היין בלבד שהוא על הגפן ועל פרי הגפן שהטוב והמטיב על היין נתקן מתחלתו כמו שאכתוב לקמן ואין חלוק בין היכא דאיכא כוס להיכא דליכא כוס אלא מיהו כבר נהגו לאומרו בכל ברכה מעין ג' מפני שהוא ברכת סעודה ואין לשנות מן המנהג.‏
Some say that one shouldn't say "Ki..." because it was only established as a summary of three [blessings]. And some say that one should say it, and even though it is a summary of four [blessings], it is only called a summary of three [blessings] which are biblical. And some say that one shouldn't say it on the blessing after grains or fruit but only on the blessing after wine, because the blessing "Hatov Vehameitiv" was initially established on wine as I will write later, though we say it when Bentching with or without a Kos. But the custom is to say it in all Summary of Three blessings...

The Rambam also notes both opinions (Brachot 8:14). The Tur (OC 208) and those after him rule that the line should be said, explaining the name in the same way as the Avudarham did.