This paper is devoted to a proof
of the rationality of the moduli space of those genus four smooth complex projective
curves which are double covers of some elliptic curve. The study of the canonical
model of a genus four curve as above allows to reduce the initial moduli problem to a
simple one in plane projective geometry; this last formulation leads to compute an
explicit representation of a certain group on a vector space and its corresponding field
of invariants.