Final answer:
The poetic device combining a short line with a four-line rhymed stanza in 'Sir Gawain and the Green Knight' is called the bob and wheel.
Step-by-step explanation:
The poetic device used in Sir Gawain and the Green Knight that is a one, two, or three-syllable word followed by four rhymed lines is known as the bob and wheel. This distinctive structure combines the 'bob,' a very short line, often only a single word, with the 'wheel,' a longer set of four lines with an ABABA rhyme scheme. The technique is characteristic of Middle English alliterative verse, especially in the works related to the Pearl Poet, the presumed author of Sir Gawain and the Green Knight. A bob and wheel adds to the rhythm and can create a memorable ending to a stanza.