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"A las cuatro" is a phrase which is often translated as "at four", and "cuarto" is an adjective which is often translated as "fourth". Learn more about the difference between "a las cuatro" and "cuarto" below.
a las cuatro()
A phrase is a group of words commonly used together (e.g., once upon a time).
b. at four o'clock
Tengo cita con mi abogado a las cuatro.I have an appointment with my lawyer at four o'clock.
cuarto, el cuarto()
An adjective is a word that describes a noun (e.g., the big dog).
A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).