Have you ever noticed that the Family Guy writers will take a 5 second bit and keep playing it for an entire minute? Like when Peter trips and falls on the sidewalk: He keeps rubbing his knee and saying "Ow" for about a minute.
The fart duet is another of these. It was funny for the first verse. Not so funny for the second. The third is just boring, and the fourth is the writer's assault on the viewers (because THEY think that's funny).