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?