What is an example of propaganda in chapter 4?

We can find an example of propaganda in the opening paragraph of this chapter. This occurs as a result of the actions of Snowball and Napoleon:

Every day Snowball and Napoleon sent out flights of pigeons whose instructions were to mingle with the animals on neighbouring farms, tell them the story of the Rebellion, and teach them the tune of "Beasts of England."

This is an example of propaganda because the purpose of sending out the pigeons is a political one. By teaching "Beasts of England" to animals on neighboring farms, Snowball and Napoleon are trying to encourage these other animals to follow suit by having their own rebellion. Remember that every lyric from Beasts of England is suited to this very purpose. Firstly, it establishes the idea of animal unity. Secondly, it depicts man as tyrannical and exploitative. Thirdly, it suggests that by overthrowing humans, animals can live in a utopia.
By sending out the pigeons, Napoleon and Snowball are spreading this message of rebellion. They want other farms to be just like theirs, just as Lenin and the other Soviet leaders spread the message of communism around the world after taking control of Russia.