Learn Italian: Queste quattro case sono nuove.

Unlike adjectives, most numbers in Italian don't change their endings to match the gender or number of the noun. The number quattro (four) will always stay quattro, whether you are counting feminine things like case (houses) or masculine things like libri (books). The main exception is uno (one), which does change!

I learned that "this" is "questo". Why is it "queste" here?

Questo changes to match the noun it points to. Because case is feminine and plural, questo (this) changes to its feminine plural form, queste (these).

Wait, doesn't the letter "-e" at the end of a word sometimes mean it's singular? Why is "case" plural?

It depends on the original ending of the word. Feminine words that end in -a in the singular, like casa, change their ending to -e to become plural. So casa (house) becomes case (houses).

Why does "nuove" end in an "-e" too?

Adjectives must match the noun they describe in both gender (masculine/feminine) and number (singular/plural). Since case is feminine plural, the dictionary form of the adjective nuovo (new) changes its ending to match, becoming nuove.