First, rank: there can be several valedictorians in an American high school; I’m estimating your “bacchilerato” rank is the same as “class rank”, ie., you’re salutatorian and share the rank with others ranked #2 or 4 sharing similar accomplishments.
Second: as SkiEurope said, DO NOT try to convert. However, your guidance counselor/headteacher/magister/form supervisor (etc, etc) would have to explain:
what percentage students get a 4.5, a 4.8-5, a 4.2, a 4, a 3.8, etc. in a typical year in your “major” in your school.
THEN
s/he would have to indicate whether that 4.2 correlates to a 4.2 for the national exams, or a 3.8, or a 4.5, indicting whether the school’s grading scale is “on level” with the national scale, stricter with grade deflation -.2, -.3, etc, or suffers from grade inflation (ie., if you have a 4.2 at your school, odds are you’ll get 3.8-4 for national exams and no higher).
COnsidering your rank, 3.8-4 sounds an approximate equivalent to an American HS GPA.