I suppose the theme of Chapter 4 is the relationship between Animal Farm and the outside world.
In this chapter, the revolution on Animal Farm is influencing the farms in the surrounding area. Other animals are wondering if they should do the same as Animal Farm did.
Because this kind of thinking is a big worry to the human farmers, they decide to do something about it. That is, they decide to attack Animal Farm to defeat the revolution and prevent it from spreading.
For later in the book, it is important to note that Snowball is the leader of the defense of the farm.
I think chapter 4 shows a quest or struggle for leadership within the fight against the humans. In Russia, which this allegory represents, and in many other countries leaders emerge from either their inspirational abilities among the people or military heroism. In this chapter we see some admirable animal fighters and we see the decorations they receive from the body of animals. One could predict that the two animals receiving honor might be viable leaders in the future: Snowball and Boxer.
This chapter also notes the growing conflict between opposing ideologies. The humans fear growth of the idea of Animalism. This same theme applies to the relationships between different countries in the real world. The ideas of democracy, communism, socialism and dictatorships all conflict. Leaders of each worry about their peoples being influenced by peoples from other ideologies.