The opera song the mediocre female singer sings who ends up dating "Lupo" (Dangerfield) to further her career (since he owns a famous restaurant), is the same tune written for Citizen Kane (1941) in which the main character, buys his "trophy" girl an opera house.
The title of this film, The 4th Tenor (2002), is a satirical reference to the Three Tenors, a popular operatic singing group consisting of José Carreras, Plácido Domingo, and Luciano Pavarotti. The group was still touring as this movie was filming, but none of the tenors make an appearance or are even mentioned in the film.