Verses 4-6 are a quotation from Isaiah. They can be stated in active form. AT: "This happened as Isaiah the prophet had written in his book" or "John fulfilled the words that the prophet Isaiah had written in his book" (See: [[rc://en/ta/man/translate/figs-activepassive]])
This can be expressed as a sentence. AT: "The voice of one calling out in the wilderness is heard" or "They hear the sound of someone calling out in the wilderness"
"Get the road ready for the Lord." Doing this represents being prepared to hear the Lord's message when he comes. People do this by repenting of their sins. AT: "Prepare to hear the Lord's message when he comes" or "Repent and be ready for the Lord to come" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-explicit]])