Learn Hardcore Ukrainian: Вона зайнята до четвертої години.

No. In standard Ukrainian, the present-tense copula is normally omitted in affirmative sentences. Вона зайнята is the natural way to say “She is busy.” You might hear є in formal, emphatic, or contrastive contexts, but it’s not needed here.

Why is it до четвертої години and not something like “до чотири години”?

Because the preposition до (to, until) requires the genitive case.

година (hour) → genitive singular: години
The ordinal number четвертий (fourth) must agree with година in case, number, and gender → genitive feminine singular: четвертої So: до четвертої години = “until the fourth hour” (i.e., until 4 o’clock).

Why is it the ordinal четвертої (“fourth”) and not the cardinal чотири (“four”)?

When you talk about clock hours with an (explicit or implicit) година (hour), Ukrainian typically uses the ordinal:

до четвертої (години) = until four o’clock. You will also hear a colloquial alternative with a cardinal:
до чотирьох (genitive of “four”) — also acceptable and common. Avoid “до чотири години” — it’s ungrammatical (wrong case and mismatch).

Can I drop the word години?

Yes. Вона зайнята до четвертої is perfectly natural in speech; години is understood from context.

Is “until four” inclusive or exclusive? Does she stop being busy at 4:00?

By default, до четвертої implies “up to 4:00,” i.e., she is free at 4:00. Context can always clarify, but the usual reading is exclusive of the endpoint.

How do I say “at four,” “from … to …,” and “by four” in Ukrainian?

At four: о четвертій (годині) (locative)
From … to …: з дев’ятої до четвертої (both genitive with these prepositions)
By four (deadline): usually also до четвертої; if you want to emphasize the latest possible time, you can add не пізніше четвертої (“not later than four”).

Why is it години and not годину?

Because до takes the genitive. години here is genitive singular of година. годину is accusative singular and would be used in different structures (e.g., duration: “for an hour”).

How is the sentence pronounced? Where is the stress?

Typical stresses:

вона́ (vo-NA)
зайня́та (zai-NYA-ta)
до
четве́ртої (chet-VER-to-yi)
годи́ни (ho-DY-ny) Also note: й in четвертої is a short “y” glide [y/j] sound: to-yi.

Can I change the word order?

Yes. Ukrainian word order is flexible.

Neutral: Вона зайнята до четвертої години.
Also fine: Вона до четвертої години зайнята. The second version front-loads the time frame for emphasis.

How do I say it in the past or future?

Future: Вона буде зайнята до четвертої (години). — She will be busy until four.
Past: Вона була зайнята до четвертої (години). — She was busy until four.

How do I say whether it’s morning or evening?

Add a part of the day (genitive):

до четвертої години ра́нку / дня / вечо́ра / но́чі Examples: Вона зайнята до четвертої години вечора.

Could this be confused with “until Thursday”?

No, the forms are different:

Until four (o’clock): до четвертої (години) — ordinal + “hour”
Until Thursday: до четверга́ — the noun четвер (Thursday) in the genitive.

How do I say “She is busy for four hours” (duration), not “until four o’clock”?

Use a duration construction:

Neutral: Вона буде зайнята чотири години. — She will be busy for four hours.
More formal: Вона буде зайнята протягом чотирьох годин. Note the difference from до четвертої (години) (until 4:00).

Are there other natural ways to say this?

Yes, depending on nuance:

Вона працює до четвертої. — She works until four.
Вона має справи до четвертої. — She has things to do until four.
Вона буде вільна після четвертої. — She’ll be free after four.
In formal notices: Працюємо до 16:00. — We’re open/working until 16:00.

If I want to say “until 4:30,” how would I do that?

Common options:

до пів на п’яту (literally “until half to five,” i.e., 4:30)
до 16:30 (24-hour time, very clear in writing)
до половини п’ятої (more formal/literary)