Answer and Explanation:
This question is about the poem "Brennan on the Moor".
The poet structured the poem in stanzas of four and only verses, because he wanted to tell a story of a hero, but do it in a musical way, where he presented a chorus that reinforces the heroism of that character, but presented stanzas that present the adventures that the hero participated.
In this case, the verses with four verses present the adventures of the hero in a more comprehensive and detailed way, while the verses of two verses, present a refraction that is repeated to reinforce the main characteristic of these heroes and reinforce the theme of the poem.