Thursday - the fourth day.
In this case, the dash (-) is the omitted present tense of the verb "to be" бути. It's like "is" in English, so the correct translation is
Thursday is the fourth day.
Isn't Thursday the fifth day?
Monday is the first day of the week in Ukraine, so Thursday is the fourth day.
And Tuesday is second day. Вторник - второй день. Wednesday is the middle day. Среда - средний день. Friday is the fifth day. Пятница - пятый день.