The paper High-speed high-security signatures by Bernstein et al. introduces the Edwards curve Ed25519.
Concerning the base point $B$, it says that
$B$ is the unique point $(x,4/5)∈E$ for which $x$ is positive
and $B$ corresponds to the basepoint used on the birationally equivalent curve Curve25519.
So I haven't found on any site information about how do they get that $4/5$ value of the $y$ coordinate in order to get a small basepoint generator.
Is there any reason or origin of that $4/5$? How they determined that $4/5$ was part of the group $2^{255} -19$?