What is a four-line stanza called?

A four-line stanza is called a quatrain. Quatrains can be rhymed or metered. They are usually separated from each other by blank lines or different indentations.

Quatrains have many possible rhyming schemes. Samuel Taylor Coleridge's poem "The Rime of the Ancient Mariner" uses the rhyming schemes ABAC and ABCB and thus is also known as a ballad quatrain. Thomas Gray's "Elegy in a Country Churchyard" demonstrates the heroic quatrain with the rhyming scheme ABAB. Alfred, Lord Tennyson's poem "In Memoriam" is an example of the enclosed quatrain that uses the rhyming scheme ABBA. Robert Frost's "Stopping by Woods on a Snowy Evening" uses AABA. Poets are not required to rhyme their quatrains, however.