Learn Russian: Четвёртый урок — история, а пятый — искусство. - The fourth lesson is history, and the fifth is art.

Because they both describe the word урок (lesson), which is a masculine noun. Ordinal numbers act exactly like adjectives and must match the gender of the noun they number. In the second half of the sentence, the word урок is dropped to sound more natural, but пятый stays masculine because it still refers to a lesson.

What is the purpose of the dashes in this sentence?

In Russian, the present tense verb for is or are is almost always skipped. When a sentence equates one noun with another (like the lesson is history), a dash is used in writing to represent that missing is. The second dash does double duty: it replaces is and acts as a placeholder for the skipped word урок.

Why does the English translation say and, but the Russian sentence uses а instead of и?

The Russian conjunction а shows a shift in focus or a mild contrast, much like whereas or while in English. Because you are contrasting two different subjects happening in two different periods, а is the natural choice over the simple additive word и.

How is the double consonant pronounced in искусство?