Learn Spanish (Spain): Son las cuatro y media.

When telling time in Spanish, the verb matches the number of hours. Because four (cuatro) is more than one, it is treated as plural, so we use the plural verb son. You only use the singular es for one o'clock (es la una).

Why is the feminine article las used before cuatro?

The article las refers to the hidden word horas (hours). Even though we drop the word horas from the sentence, the word for "the" still has to be feminine and plural to match it.

Why is it media ending in "a" instead of medio?

Just like las, the word media (half) refers back to the hidden word hora (hour). Because hora is feminine, we use the feminine form media.

Could I say son las cuatro y treinta instead?

Yes! Just like in English you can say "four thirty," you can say son las cuatro y treinta. But using y media for "half past" is very common and sounds much more natural for everyday time-telling.