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.