What are the differences between the characters of Frederick and Pilkington and between the farms they run?
In Chapter IV, Orwell describes Mr. Frederick as a tough, shrewd man who was always involved in lawsuits and had a reputation for driving hard bargains. His farm was called, appropriately, Pinchfield. It was small and well maintained. Orwell describes Mr. Pilkington as a "gentleman farmer" who neglects his property and spends much time hunting and fishing (not unlike the English country squires of Orwell's day). Understandably, his farm, called Foxwood, is overgrown and "in a disgraceful condition." They both hate each others, but they cooperate in the campaign against the animals at Animal Farm because they are afraid those animals will set an example which will lead their own animals to revolt and cause them to lose their farms. Radicals would describe both men as "reactionaries." Mr. Frederick probably represents the petit bourgeois, the nouveau riche, while Pilkington probably represents the landed gentry, "old money," in this allegorical story. Both men and their farms are covered in the first two pages of Chapter IV.
The main thing to remember here is that Frederick is meant to represent Germany (and Hitler specifically) while Pilkington is meant to represent England (not a specific leader). Their personalities are similar to what those countries were like around the 1930s.
Frederick is an orderly man who has his farm just the way he wants it. It is all neat and tidy and he works very hard at it. He is a disagreeable man, though, and is always getting involved in lawsuits with his neighbors.
By contrast, Pilkington takes it easier. His farm is a bit overgrown in place. He would rather go fishing and hunting than work. But he is a nicer guy.