Would line 4 in 'Bushed' be considered part of the first stanza or part of the second stanza?
The line in question, from Earle Birney's poem "Bushed," is "yet he built a shack on the shore." With each of the sites I visited, the line is the first line of the second stanza. Based upon your question, and the placement of the line always existing in the second stanza, I believe that the question is posed given the lack of punctuation in the poem. For some poets, grammatical rules do not exist. Punctuation need not be used.
"Bushed" is written in free-verse which means the poem does not adhere to any specific metered form. The poem lacks punctuation and, therefore, can be confusing for some readers. To denote stanzas, a poet places a space between the last line of one stanza and the first line of the next. (A stanza is a grouping of lines.)
Here is the line in question as it appears in the poem:
He invented a rainbow but lightning struck it
shattered it into the lake-lap of a mountain
so big his mind slowed when he looked at it
Yet he built a shack on the shore
Given the line in question comes after the space, the line exists as part of the second stanza.