Learn Spanish (Latin America): Ya son las cuatro menos veinte.

When telling time in Spanish, you use the plural verb son (they are) for all hours except one o'clock. This is because you are talking about multiple hours. For one o'clock, you use the singular es la una, but for four o'clock, it must be son las cuatro.

Why do we use the feminine article las before cuatro?

Time in Spanish is always feminine because it refers to the word hora (hour) or horas (hours), which are feminine. Even though the word horas is left out of this sentence, the feminine plural las is still required.

Does cuatro menos veinte literally mean "four minus twenty"?

Yes, it does! In English we often say "twenty to four", but in Spanish, once you are past the 30-minute mark, you usually state the upcoming hour and subtract the remaining minutes using the word menos (less or minus).

Could I put ya at the very end, like we say "It is twenty to four already" in English?

You could, but placing ya (already) at the beginning of the sentence or right before the verb is much more natural and common in Spanish.