In 1984, what is Orwell's purpose in having all humans in chapter four so unpleasant?
To a great extent, Orwell's construction of the humans in this chapter is done to enhance the "us against the world" element that forged the Communists to victory in the Russian Civil War following World War I. The human collusion to attempt to put down the animals represents the Allied Forces who entered into Russia and supported the "Whites" in the Russian Civil War from 1918- 1920. The humans cannot be displayed in a manner that is benevolent towards the animals because it is this lack of support that allows the animals to unify and fight as a unit. Heroes like Boxer and Snowball becomes dominant forces in this struggle because of their total commitment to the animal cause against the humans. The human beings cannot be depicted as anything other than unpleasant because they represent the Allied forces who lacked the insight to understand the Russian predicament of the time and simply presumed that their mere presence would spell out victory. Another reason why the humans are depicted in such an unpleasant manner could also connect to the "historical unpleasantness" the Allied forces demonstrated during the Russian Civil War:
At about the same time, the Russian Civil War (1918-1920) broke out between the Communist “Reds” and anti-Communist “Whites.” The allied powers, including the United States, Great Britain and France (who were still at war with Germany), and Japan invaded Russia and occupied Russian territory. After the defeat of Germany, the Allied forces remained in Russia and aided the Whites against the Reds. Coming under attack by combined Russian troops, the foreign forces withdrew in 1919, and victory in the Civil War went to the Communists. From it all, Trotsky emerged as a powerful leader, the main architect of the new Red Army.
It is fairly "unpleasant" that the Allied forces could enter a civil war in Russia, support one side, and then simply abandon that side when it begins to take pressure and setback.