Why is the number "cuatro" not declined?

[Yoodle15]

"El viaje de Raquel tiene une demora de cuatro horas."

Gracias por adelantado. :)

[Perikles]

"El viaje de Raquel tiene une demora de cuatro horas."The numbers un(o)/-a and cientos/-as agree in gender with the following noun. These are the only numbers that decline. If cuatro were to agree, it would not exist as cuatro, only :bad:cuatros/-as. :rolleyes:

(Ordinal numbers are nouns or adjectives, so their change is called declination. Only verbs conjugate). :)

[Yoodle15]

Gracias! :)