Egypt forward Marmoush, signed from Eintracht Frankfurt in January, scored his first two goals for City in the space of five minutes.
He added a third before the break as the defending champions clicked into gear at the Etihad Stadium.
James McAtee volleyed home City’s fourth from close range as Pep Guardiola’s side bounced back from their midweek home Champions League defeat to Real Madrid.
The win is City’s biggest home league win of the season. (dpa/NAN)